Method for balancing circuit voltage

ABSTRACT

Various techniques are employed alone or in combination, to reduce the levelized cost of energy imposed by a power plant system. Solar energy concentrators in the form of inflated reflectors, focus light onto photovoltaic receivers. Multiple concentrators are grouped into a series-connected cluster that shares control circuitry and support structure. Individual concentrators are maintained at their maximum power point via balance controllers that control the flow of current that shunts this series connection. DC current from clusters is transmitted moderate distances to a centralized inverter. The inductance of transmission lines is maximized using an air-spaced twisted pair, enhancing the performance of boost-type three phase inverters. Cluster outputs are separate from individual inverters in massively interleaved arrays co-located at a central location. Step-up transformers convert inverter voltages to grid voltages, and small transformers provide isolation and voltage step-up only on receiver-to-receiver imbalance currents, typically &lt;20% of the total current.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/782,932, filed May 19, 2010, which claims the benefit of U.S.Provisional Application No. 61/179,606 filed May 19, 2009, which isincorporated herein by reference in its entirety for all purposes. Theapplication is also related to the following applications, each of whichis incorporated by reference in its entirety herein for all purposes:U.S. patent application Ser. No. 11/844,888 filed Aug. 24, 2007; U.S.patent application Ser. No. 11/843,531 filed Aug. 22, 2007; U.S. patentapplication Ser. No. 11/844,877 filed Aug. 7, 2007; U.S. patentapplication Ser. No. 11/843,549 filed Aug. 22, 2007; U.S. patentapplication No. 61/299,124 filed Jan. 28, 2010; and U.S. patentapplication No. 61/310,228 filed Mar. 3, 2010.

BACKGROUND

Solar energy is abundant and sustainable. However, using solar energy topower an electrical grid may offer certain challenges.

For example, photovoltaic cells generally produce a maximum power at aparticular voltage and current that depends on the properties of thecell and the amount of illumination. Away from this maximum power point,the conversion efficiency of the cell drops.

A utility-scale power plant may comprise such cells numbering in thethousands, deployed across square kilometers. Given this large scaleconfiguration, it may be difficult to operate an entire plant at peakefficiency.

In addition, the output from photovoltaic cells is typically processedto produce an alternating current to output onto the electrical powergrid. This can be difficult to manage with such a large number ofdiscrete PV cells.

Finally, a solar power plant must operate under a range of non-idealconditions. Examples of non-ideal conditions include a lack of fullsunlight, and possible outages on an electrical power grid.

Accordingly, embodiments of the present invention relate to acost-optimized architecture for a photovoltaic power plant that canoperate at or near its maximum production efficiency. Embodiments of thepresent invention may continue to function under adverse conditions,such as grid outages and lack of full sunlight.

SUMMARY

A power-plant power system architecture employs techniques to reduce thelevelized cost of energy imposed by the power system, the elements ofwhich may be unconventional. In one embodiment, a solar energyconcentrator in the form of an inflated reflector, focuses light onto ahigh-concentration photovoltaic receiver. A plurality of theseconcentrators are grouped into a series-connected cluster that sharescontrol circuitry as well as support structure. Individual concentratorsare maintained at their maximum power point via balance controllers thatcontrol the flow of current that shunts this series connection. DCcurrent from clusters must transmit moderate distances, e.g., 300-1000 mto a centralized inverter. The inductance of transmission lines ismaximized using an air-spaced twisted pair, enhancing the performance ofa boost-type three phase inverter. The outputs of clusters are keptseparate to individual inverters in a massively interleaved arrayco-located at a central location. Step-up transformers convert frominverter voltages to grid voltages, and small transformers provideisolation and voltage step-up only on receiver-to-receiver imbalancecurrents, typically <20% of the total current. These massive inverterarrays can be pre-assembled and tested in rack-mounts in standardshipping containers for convenient deployment.

These and other embodiments of the present invention, as well as itsfeatures and some potential advantages are described in more detail inconjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A & 1AA are conceptual and physical illustrations of anarrangement of balancing circuitry according to an embodiment of thepresent invention, which supports a cluster of N receivers.

FIG. 1B shows a conventional cluster of series-connected receivers.

FIG. 1C shows a conventional approach to handling power imbalances amongclusters of receivers.

FIG. 1D shows a conventional cluster in which maximum-power-pointtracking switchers are series connected.

FIG. 1E shows an alternate arrangement of balancer circuits which canpass current bi-directionally between each receiver without the need fora direct connection to each other or a connection to a power bus.

FIG. 2A shows a balancer circuit arrangement in which the balancercircuit for each receiver is separate.

FIG. 2AA shows a simplified schematic diagram of a bidirectionalisolated flyback converter used for bi-directional power flow.

FIG. 2AB shows a simplified schematic diagram of a half-bridgebidirectional converter used for bi-directional power flow.

FIG. 2AC shows a simplified schematic diagram of a bi-directional Cukconverter used for bi-directional power flow.

FIG. 2B shows an alternative arrangement or balancer circuitry in whicha single multiple-tap transformer is employed.

FIG. 2C shows a balancer circuit that employs a passive rectifierbridge.

FIG. 2D shows a balancer circuit that employs a transformer that hasisolated, coupled windings.

FIG. 2EA shows a simplified schematic diagram of a unidirectionalisolated flyback converter.

FIG. 2EB shows a simplified schematic diagram of a unidirectionaltwo-switch forward isolated converter.

FIG. 2EC shows a simplified schematic diagram of a unidirectionalisolated Cuk converter circuit.

FIG. 3 shows an illustration according to the present invention, of anembodiment of a power plant layout.

FIG. 3A shows a simplified view of a power plant according to anembodiment of the present invention.

FIGS. 3AA-3AE show arrangements of cluster voltages according toembodiments of the present invention.

FIG. 4A shows a diagram of a pair of DC transmission lines.

FIG. 4B shows a cross-sectional view of the spacing of an insulatedtwisted pair maintained by a polymer web.

FIG. 4C shows a cross-sectional view of an alternative cross-section ofan insulated twisted pair which minimizes polymer use.

FIG. 5A shows an embodiment of a physical layout of die on a receiversubstrate.

FIG. 5B shows a schematic diagram of a electrical circuit in a receivershowing an assortment of design elements aimed at improving off-designperformance.

FIG. 6 shows such a cluster of collectors.

FIG. 7 shows a schematic block diagram of an embodiment of amaster/slave arrangement of balancers.

FIG. 8 shows a schematic diagram of an embodiment of a balancer slaveaccording to the present invention.

FIG. 9A shows a schematic diagram of an interleaved inverter systemaccording to an embodiment of the present invention.

FIG. 9B shows an embodiment of an inverter system that provides forswitch synchronization daisy-chaining via circuitry.

FIG. 10A shows a schematic diagram of an embodiment of an inverter slaveaccording to an embodiment of the present invention.

FIG. 10B shows a diagram of an embodiment of an inverter slave that usesa daisy chain of handshake signals to perform soft switching,reduced-stress switching, low-noise, switching, or other switchingenhancements.

FIG. 11A shows a UPS associated with inverters according to anembodiment of the present invention.

FIG. 11B shows an electrical connection arrangement for reducingsecondary damage from a failed switch.

FIG. 11C shows an alternate circuit for reducing secondary damageresulting from an increase in negative common voltage substantiallyabove the three-phase plant voltages.

FIG. 12A shows a schematic circuit diagram of a low-side switch modulecomprising a low side driver module and a switch module.

FIG. 12B shows a schematic circuit diagram of an embodiment of ahigh-side switch module.

FIG. 13A shows an embodiment of a switch module.

FIG. 13B shows the switch module illustrated in FIG. 13A with its coverremoved.

FIG. 13C shows a view of the base side of the switch module cover.

FIG. 14A shows a simplified mechanical drawing of the assembly of switchmodules, low-side modules, and high-side driver modules as would beplaced into an inverter slave housing.

FIG. 14B shows an arrangement of low-side and high-side driver modulesand switch modules inserted into an inverter slave housing according toan embodiment.

FIG. 14C shows a simplified internal assembly of an embodiment of aninverter slave.

FIG. 15A shows a top view of an inverter slave according to anembodiment of the present invention.

FIG. 15B shows a shows a back side view of an inverter slave accordingto an embodiment of the present invention.

FIGS. 16A and 16B respectively show isometric top front and back viewsof an embodiment of an inverter slave engaged with its active coolingmodule.

FIGS. 17A-17D show components of an embodiment of a coolant module.

FIG. 17E shows an alternate interface that mates the cooling module andcooled elements

FIG. 17F is an expanded view of the cooling plate and cooled plateinterface arrangements illustrated in FIG. 17F.

FIG. 17G shows an alternate interlocking interface between the coolingplate and cooled elements.

FIGS. 18A and 18B show the cold side of a splitter plate.

FIGS. 18C and 18D show the same patterns on the hot side of the splitterplate.

FIG. 18E shows how the cooled components lie physically with respect tothe patterns of jets in the splitter plate.

FIG. 19 shows a ten-by-ten array of coolant modules assembled into aninverter heat exchanger.

FIGS. 20A-20D show an embodiment of an assembly of inverter slaves,cooling system, motherboard, and back panel, fitting together.

FIG. 21 shows a mechanical drawing of a housing structure according toan embodiment of the present invention, that contains the assemblyillustrated in FIGS. 20A-20D.

FIG. 22 shows the assembly illustrated in FIGS. 20A-20D disposed in ahousing without a front panel attached.

FIGS. 23A-23E respectively show a top-front isometric view, a frontview, a back isometric view, a back view, and side view of the inverterassembly 2300 with front-panel attached.

FIG. 24 shows an embodiment of the interleaved inverter containing amotorized traverse to programmatically operate arrays of mechanicalswitches.

FIG. 25 shows a schematic diagram of a communications and controlnetwork according to an embodiment of the present invention.

DETAILED DESCRIPTION

As used herein the “microcontroller” refers to a digital system that canperform a programmed function, e.g., a standard microcontroller, FPGA,CPLD, microprocessor, computer, ASIC, system on a chip, or compositeassemblies of parts that can perform a function. As used herein, aswitch refers to something whose conductivity can be made to change,e.g., MOSFETs, IGBTs, fets, bipolar transistors, SCRs, mechanicalswitches, mechanical and solid state relays, contactors, an IGBT inparallel combination with a free-wheeling diode such as a fast-recoverydiode, a series combination of a MOSFET and blocking diode orientedoppositely to the MOSFET body diode, a series combination of MOSFETsconnected such that their body diodes are oppositely oriented, and thelike.

Embodiments of the present invention relate to a concentratedphotovoltaic-based power plant for utility-scale electricity generation.An architecture of the plant is driven by at least the following twoconsiderations. First, the plant is designed to be readily scalable toproduce power levels relevant to global energy consumption.

Second, the plant is designed to achieve the minimum possible levelizedcost of energy. This minimum cost includes various aspects of producingpower, including but not limited to initial capital costs, installationcosts, maintenance costs, and consumable costs.

According to embodiments of the present invention, power plants aredesigned that can be routinely upgraded by improved firmware andsoftware, and managed obsolescence of rapidly evolving components.Certain cost savings are based on making maximal use of phenomena andmaterials that are economic externalities, for example the use of airand soil as structural elements. Other cost savings may be achieved byforming a rigid concave reflector shape of a thin film under airpressure. The architecture of the power distribution system can utilizea similar cost-saving philosophy.

Particular embodiments of the present invention utilize inflatedreflective films, or “balloons”, to concentrate light onto photovoltaicreceivers. Additional details describing various embodiments ofinflatable solar concentrator balloon methods and apparatuses, which maybe suited for use in accordance with the present invention, can be foundin U.S. patent application Ser. No. 11/843,531, which is published asU.S. Patent Publication No. 2008/0047546 and U.S. Provisional PatentApplication No. 61/299,124, which are both incorporated by reference intheir entirety for all purposes. Additional details describing variousembodiments of receiver structures, which may be suited for use inaccordance with the present invention, can be found in U.S. patentapplication Ser. No. 11/844,888, which is published as U.S. PatentPublication No. 2008/0135095, which is incorporated herein by referencein its entirety for all purposes.

Each mirror may reflect light from an approximate 2.25-3 m diameter areato a secondary optic roughly 200 mm in diameter. This secondary opticdistributes and further concentrates light by a factor of approximately2-3, onto a dense array of photovoltaic die, while providing passiveoptical compensation for pointing-angle errors. Additional detailsdescribing various embodiments of secondary optic structures andinterconnection of die can be found in U.S. patent application Ser. No.12/720,429, which is incorporated by reference in its entirety for allpurposes.

According to certain embodiments, dies may be series-connected insubstrings. These substrings can be joined together in series and/orparallel combinations to provide passive electrical compensation forpointing-angle errors.

FIG. 5A shows an embodiment (500) of a physical layout of die (e.g.,502) on a receiver substrate (504). Some sequences of adjacent die maybe series connected in strings, for example 506.

In some embodiments, strings may be paralleled with others to providesubstantially the same total power output when pointing is non-ideal.For example, string 506 could be paralleled with string 508, 510, or512.

In some embodiments, a region of die 514 are substantially seriesconnected. In some embodiments, compensation may be provided near thecenter, for example via paralleling strings 516 and 518, 520, or 522.

FIG. 5B shows a schematic diagram 530 of a electrical circuit in areceiver showing an assortment of design elements aimed at improvingoff-design, e.g., mis-pointed, receiver performance. Solar cell die,e.g., 532, are connected in series chains, e.g., 534 into a string 536.

Element 538 may be a diode, typically a Schottky diode because of itslow forward voltage drop, that provides an alternate current conductionpath when the string current is greater than that supported by thephotocurrent produced by the die. Such a case can arise when one or aplurality of die in a string are under-illuminated.

A number of die in a string bypassed by a Schottky diode may bejudiciously chosen, and this number may vary from one string (e.g., 536)to another (e.g., 540). Considerations in this choice can include butare not limited to including the reverse breakdown voltage of the solarcells, the receiver-location-specific variation in illumination, andothers.

In some embodiments, a plurality of strings, e.g., 542 and 544 may beparalleled, and the combination are protected by a diode 546 across theterminals of the strings 548. In certain embodiments, individual die ora substring of die may be paralleled via connections 550 and bypassdiode(s) inserted across the terminals of the strings 552. In someembodiments, bypass diodes, e.g., 554, may be arranged to reduce thecumulative voltage drop of a plurality of bypass diodes if multiplediodes are forward biased. The voltage between receiver terminals 556and 558 is the receiver output voltage.

It may be desirable to connect a capacitor across this connection in alocation that is physically close to a receiver. In concentratedphotovoltaic receivers, this connection may be performed at a safedistance from the receiver, away from heat and high illumination levels.

The size of capacitance can be chosen judiciously according to one ormore considerations. A smaller capacitor is typically less expensivethan a bigger one, but isolates the receiver from current ripple arisingfrom switching circuitry less. This current ripple may reduce efficiencyby causing the receiver to depart periodically from itsmaximum-power-production region.

An excessively large capacitor may cause problems with high inrushcurrents in switching circuits, such currents being far larger thancould be sustained by the receiver. This could thereby increasepotential current stresses, and possibly requiring additional protectioncircuitry and snubbing. One element of certain embodiments according tothe present invention is the use of a capacitor that is small enough toobviate or mitigate source-side over-current protection.

An objective of the secondary optic/receiver design is to provide peakpower output for the largest possible pointing errors. In somesituations, it may be appropriate for a receiver to produce more thanone voltage or current output. Such a configuration avoids excessivelydividing the receiver voltage by paralleling to a common voltage thatall substrings can support.

According to certain embodiments, a target power for each receiver is O(600-1000 W) with a voltage of O (100 V). Using silicon solar cells,this corresponds to approximately 200-400 individual die, given someparalleling of strings. Throughout this description some values arepresented using “0,” which stands for “on the order of” For example O(600-1000 W) stands for on the order of 600 W to 1000 W.

In certain plant designs according to embodiments of the presentinvention, concentrators are grouped into a cluster, for example 8balloons that share a common support and tracking structure andinflation controller. Additional details describing various embodimentsof balloon support and/or tracking structures can be found in U.S.patent application Ser. No. 11/844,877, which has been published as U.S.Patent Publication No. 2008/0168981 and U.S. Provisional PatentApplication No. 61/310,228, which are both incorporated by reference intheir entireties herein for all purposes.

FIG. 6 shows such a cluster 600 of collectors. Elements 602 areconcentrators, element 604 is a structure that supports theconcentrators. Elements 606 are receivers that convert light that fellon the mirrors of 602.

In some embodiments, each cluster produces approximately between 6-8 kW.The output or outputs of individual concentrators are series connected.However, the maximum power of each concentrator is expected to varysignificantly (e.g., up to 30%) over the life-span of its reflector.This variance can depend on such factors as the model of the reflector,and variations in the receiver.

In order to maximize the total power produced by the cluster, eachconcentrator can be operated near its maximum power point, whichrequires excess or deficit currents to be drawn or supplied individuallyto each concentrator by a balance circuit. Where the strings are seriesconnected, this balance circuitry only needs to supply or draw imbalancecurrents (for example <30% of the total load), thereby providing thedual advantage of requiring lower-rated circuitry and dramaticallyreducing the dependence of the overall efficiency on the efficiency ofthe balance circuitry.

Details describing various embodiments of interconnection systems forsolar energy modules and ancillary equipment can be found in U.S. patentapplication Ser. No. 11/843,549, which has been published as U.S. PatentPublication No. 2008/0057776, and are incorporated by reference in theirentirety herein for all purposes. The DC voltage from each cluster maybe transmitted on high-inductance twisted pairs of aluminum wires havingoptimized cross-sectional area. The length of these individual wires ina power plant may be between about 300-1000 m, because of the relativediffuseness of the solar power resource. With a 0.5 cm air-spaced helix,it is practical to obtain inductances of several hundred micro-Henriesover transmission lines of this distance, reducing the need for inputfilter inductors for properly chosen inverter architectures (andobviating them in some arrangements).

The DC power from each cluster (or in some cases small groups ofclusters) goes to individual three-phase inverter circuits that aremassively interleaved. The DC power from clusters may not be combined inorder to reduce the wire conductor count. In general, a cost of thesmall amount of extra insulator material to keep the lines separate, ismore than offset by the elimination of power-sharing concerns among theinterleaved inverters.

A 1 MW plant can include approximately 200 clusters, thus O (200)interleaved inverters are combined to produce precision sinusoidaloutputs with minimal filtering. Inverters may be co-localized,simplifying maintenance and high-frequency coordinated interleaving.These inverters are preferably arranged in a factory pre-assembled andtested rack system in a standard shipping container.

According to certain embodiments, the primary power path of theseinverters is completely transformer-less. This is because transformersare expensive in up-front capital costs, produce significant parasiticelectrical losses, and require ongoing maintenance.

The function of electrical system isolation, may be served by the mainpower plant step-up transformer at the point of electricalinterconnection with the grid. In specific embodiments, this main planttransformer can comprise four single-phase transformers, with threenormally in service and the fourth serving as a plant spare. Thetransformers will also be co-located with the inverter (within ˜30 m).

This above description is specific to a particular embodiment. However,the approaches to power extraction, and the hierarchy of receiver,cluster, and centralized inverter described herein, are widelyapplicable to high-performance, cost effective concentrated and one-sunphotovoltaic energy conversion.

Design of a power plant according to embodiments of the presentinvention, may be guided by one or more of the following principles.

Efficiency may be pursued only as cost justified. This is due to themarginal cost of increased DC power production of a technology-specificcost per Watt, for example less than about $0.50-5.00/W.

The greatest result may be achieved utilizing the fewest resources. Forexample, soft switching may be employed as much as possible withoutadding cost or complexity. As another example, sophisticated timingtechniques and interleaving may be exploited to achieve the bestperformance out of hardware.

Complexity in software may be favored over complexity in hardware.However, it is important to ensure that the system is tolerant ofmicrocontroller overhead tasks, and to reduce risk by judicious use ofanalog circuitry (for example managed one-cycle control).

Where possible, switching is performed only on imbalance currents ormismatch voltages. Active operation is only on the margins.

Long wire runs should be DC at the highest practical voltage, withmanaged Electromagnetic Interference (EMI)/Radio Frequency Interference(RFI) and inductance. For example, a twisted pair with controlledspacing and/or flattened twisted pair is used. A spacer could beformulated with material to augment inductance with low loss, if costjustified. Avoid placement close to ferro-metals unless specificallydesigned for low eddy current/hysteresis losses so interconnectinductance is more nearly ideal.

AC runs could follow the shortest possible paths to the voltage-step-uptransformers (for example <10 m).

Large voltage step-up/step-down operations can be performed only once,and only where a transformer is otherwise required, for example forisolation. A turn ratio on the isolation transformer can be employed tooptimize efficiency.

Chopper/switcher circuit architectures may be utilized that benefit fromthe inductance of the interconnect wires. An example is where theinductance of the interconnect wires replaces or is in series with, adiscrete inductor.

Where practical, DC currents from separate sources are not combined.Rather, the DC currents are run parallel to separate switching circuitryin order to avoid issues of power balancing.

Separate switching circuitry may be massively interleaved. Thisinterleaving is finely timed in order to reduce switching stresses.

Separate switching circuits may be grouped to one location, and in somecases to one enclosure. Such grouping can minimize componentduplication, facilitate interleaving and high-frequency communicationsand coordination, minimize enclosure cost, minimize maintenance costs,minimize enclosure and interconnection costs, etc.

Water cool switches, capacitors, and/or inductors can be used to save onheat sinks, and extend component life, reduce size, eliminate fans andopenings to the environment.

Elements that tend to fail may be grouped in minimum units, e.g., singleIGBT or MOSFET+driver, and these groupings are made easily and safelyswappable in the field. These are consumable parts, and should beminimized along with the cost of replacing them. The system can also bedesigned to eliminate throwing good parts away with bad ones because ofco-packaging, for example the case with IGBT “six-packs.”

Elements are packaged soundly to prevent secondary damage orcontamination on failure. An example of this is packaging to containexploding electrolytics or burning IGBTs.

A hybrid of analog and digital control may be used for the best dynamicresponse and pseudo-static optimizations. This is referred to as managedone-cycle control (mOCC).

The system may be designed to be able to oppose non-ideal gridwaveforms, such as departure from sinusoidality and to controlproduction of imaginary power. This opposition could require theattention and intervention on a sub-cycle or cycle-to-cycle basis of amicrocontroller. This behavior may be useful for solar power plants tobecome majority power producers.

Where possible, systems are designed for unconditional safety in theevent of microcontroller malfunctions. Ideally, the systems shouldcontinue to work at a reduced performance level.

Massive interleaving is favored over large passive components to producehigh-quality output.

The interleaved inverter may be designed to degrade gracefully wheninverters fail or clusters are taken offline.

Essential communications can be performed using wired connections,preferably using RS485 physical layers or similarly mature and robustlinks. Wireless links may be used for non-essential control ormonitoring or for sparse auxiliary equipment such as weather stationsand video surveillance/monitoring/event troubleshooting. Barring newdevelopments, optical links should be used only if necessary because ofcost concerns.

Microcontroller firmware can be updated from a central location, forexample via a plant-local RS485/Ethernet network.

Remote debugging and control may be implemented over a plant-localnetwork. For example, the plant network can comprise RS485/Ethernet andpreferably via a non-local network such as the internet.

All elements of the power system can be designed to survive lightningstrikes to the plant without damage, via lightning-rods, grounding, andappropriate transient suppression at entry and exit points of shieldedenclosures.

Receiver Design

The receiver may contain a ˜200-mm diameter, one-layer,high-heat-transfer, printed circuit board bearing a radial array ofrectangular silicon solar cell die attached to printed circuit tracesvia eutectic solder and aluminum wire or ribbon bonds. The individualdie may be series connected into a number of strings. The board is keptcool via impingement of numerous immersed jets of cool water on the backside of the printed circuit board that is electrically insulated fromthe front circuitry by a thin thermally conductive ceramic dielectric.

The power from strings is carried via multiple conductors from thisprimary printed circuit board to a nearby conventional printed circuitboard, for combining in parallel and series arrangements to reduce theeffect of concentrator mis-alignment on power output. An example isshown in FIGS. 5A and B. It is possible that the final receiver suppliesmore than one voltage/current through separate outputs if it isimpractical to achieve enough passive electrical alignment-errorcompensation otherwise. This secondary board may also contain filtercapacitors, bypass diodes, active switching circuitry, and sensingcircuitry. Water cooling is available for this circuit board, if needed.

This board can be located at the interior of the concentrator at heightsthat make servicing difficult. Therefore it may not be an ideal locationfor components that require periodic replacement, such as electrolytics.

In one embodiment, the target voltage for the receiver is O (100 V @ 8A). This voltage may be produced by a series and parallel combination of˜200 to 400 silicon solar cell die illuminated at several hundred sunsconcentration. For minimal support structure stiffness requirements, themass of the receiver should be kept at a practical minimum.

The following summarizes one or more characteristics of particularembodiments of a receiver.

-   -   The receiver produces ˜O (100 V @ 8 A).    -   The physical receiver comprises an approximately 200-mm diameter        one-layer die-mount board with conventional secondary PCB for        string combining and/or active power circuitry and/or sensing        circuitry.    -   The receiver employs passive electrical compensation coupled        with passive optical compensation.    -   The receiver provides for active compensation of imbalance        currents/voltages of substrings to approximate MPPT across        in-homogeneously illuminated receiver.    -   The receiver includes high-frequency filter capacitors,        preferably on a secondary, conventional, printed circuit board.    -   The mass of the receiver should be as low as practical.    -   Mounting low-frequency filter capacitors on the receiver may not        be favorable because of thermal stresses, extra mass, and        difficulty of replacement.    -   Water cooling is available for electronics at the receiver.

Cluster Design

A cluster comprises multiple concentrators in proximity, for example asshown in FIG. 6. In certain embodiments, a cluster comprises O (8)concentrators. Clusters of concentrators share support structure, tracktogether, and share control circuitry in an easily serviced enclosure,typically within about 8-10 m of wire distance of the furthestreceivers.

To reduce the number of series-connected die to a minimum for cost andreliability considerations, in certain embodiments the receivers maysupply only O (100 V). These modules may be connected in series toobtain a voltage O (600-1000 V) at ˜8 A for efficient plant-scale (e.g.,300-1000 m) DC-transmission to our inverter circuitry.

In particular embodiments, circuitry is needed to shunt currents toand/or from each receiver to ensure each receiver is individually poweroptimized. This circuitry is referred to as “balance circuitry.”

An analogous approach could apply to paralleled receivers, in which apower supply/sink provides a boost voltage, or extracts a voltage fromeach receiver so that each receiver operates at its maximum power pointat a common output voltage. Such an arrangement might be particularlyadvantageous for series of high-voltage solar cells, such astriple-junction cells, which more readily reach high voltages.

FIGS. 1A & 1AA are conceptual and physical illustration of anarrangement 100 of balancing circuitry according to an embodiment of thepresent invention, which supports a cluster of N receivers 102. Thereceivers are series connected via conductors 104 through which a commoncurrent i₀ flows.

An array of “Balancers” 106 either unidirectionally draws current 108,unidirectionally supplies current 110, or bidirectionally passes current112 through shunt paths. This allows each receiver to supply a totalcurrent different from i₀, such that each receiver independentlyoperates at its maximum power point.

The output voltage 114 relative to the current return 116 is the sum ofthe receiver voltages. The output current is the sum of the commoncurrent i₀ and the individual Balancer currents i_(jb), which could bepositive or negative depending on the design of the Balancers.

The specific balancing arrangement shown in FIG. 1A represents just oneembodiment, and alternatives are possible. For example, otherembodiments may employ parallel input connections and imbalance-voltagebalancing, drawing and supplying power to or from a separate DC or ACline, etc.

Most of the gain of the Balancer arrangement lies in operating only oncorrecting or adjusting imbalances, rather than adjusting the fullreceiver power. The particular arrangement of the embodiment of FIG. 1Amay minimize the cost of routing imbalance power through the plant,relative to some of the alternates just mentioned.

The Balancer system according to an embodiment of the present inventionshown in FIG. 1A, may be contrasted with conventional configurations.For example, FIG. 1B shows a conventional cluster 130 ofseries-connected 134 receivers 132. In this approach, all receivers mustpass the same current, producing the maximum possible power only if thereceivers and their illumination is suitably matched.

FIG. 1C shows a conventional approach 150 to handling power imbalancesamong clusters of receivers 152. Separate switching circuits 154 operateso as to draw the maximum power 156 from each receiver, producing acommon output voltage V_(0b) to paralleled outputs across 158 and 160.

Alternatively, FIG. 1D shows a conventional cluster 170 in whichmaximum-power-point tracking switchers 172 are series connected,producing a common output current. These switchers can be converters,producing a DC output or inverters, producing one or more phases of ACoutput.

In the case of either of the convention approaches shown in FIG. 1C or1D, the switching circuit operates on 100% of the output power of thereceiver, requiring correspondingly large and expensive components andmust be highly efficient to be justified if expected imbalance currentsare small. As a result, the extra power harvested by circuits in FIG. 1Cand FIG. 1D is at best marginally cost justified over theinterconnection approach in FIG. 1B. In contrast, the Balancer approachin FIG. 1A overcomes these cost issues without sacrificing output power.

The design 180 in FIG. 1E shows an alternate arrangement of balancercircuits which can pass current 182 bi-directionally between eachreceiver without the need for a direct connection to each other or aconnection to a power bus. Instead, balance currents are passed via atleast one transformer 184 which inductively couples outputs of balancers186.

According to embodiments of the present invention, the Balancers may bedesigned to support a maximum current imbalance that is considerablylower than the maximum expected series current i₀. In normal operation,it would be unlikely for receivers to exhibit more than a certainvariation in output power relative to each other, for example betweenabout 5-20%. The Balancer may be designed for best efficiency in thisnormal range and/or a reduced efficiency up to a maximum power rating.This de-rating of the balance circuit can considerably reduce cost.

Moreover, the required efficiency of the balance circuit is much lowerthan if it operated on the full power output of the receiver. Forexample, if a Balancer has an efficiency of 80% while Balancing 20% ofthe receiver current, the overall power efficiency, assuming no seriesconnection losses, is a respectable 96%. The reduced efficiencyrequirements translate to further cost savings.

If maintaining the maximum power point requires exceeding the maximumpower limits of a Balancer, the weakest-performing active receiver in acluster could be shut down. Balancers that draw excess receiver currentas in 108 must be coordinated (for example via communications orhandshaking) because a maximum power condition at a Balancer associatedwith the top-performing receiver can prompt the shut-down of thebottom-performing receiver. In some cases, especially for larger N,shutting down a weak-performing receiver may be prompted by a netreduction in output associated with inefficiencies in the Balancecircuits.

In the event of shut down, the Balancer may 1) actively short-circuitits associated receiver (in the manner of a synchronous rectifier switchusing mechanical relays or solid state switches), 2) passively bypassthe associated receiver with a diode, or 3) perform no operation if itsassociated receiver has acceptable bypass elements. If the receiver usessynchronous rectification, it may be paralleled with passive bypasscircuitry so the Balancer may periodically deactivate the bypass switchto check to see if the receiver performance has changed. It may beadvantageous to choose a balance-circuit architecture that utilizesswitches that can short-circuit the receiver when kept turned on.

The power used by a balancer circuit can be supplied in whole or in partby the voltage V_(j) from its associated receiver or by the voltageΣV_(j) from the cluster or from an alternate power source. The powersource for critical balancer circuitry (such as microcontrollers), mustbe designed and managed judiciously to ensure devices do not “brown out”or enter indeterminate states, and to ensure that the system remains incontrol and responsive as needed, regardless of energy production fromits associated receiver and to a lesser extent, its cluster.

Receivers may be routinely inoperative, for example during and awaitingconcentrator maintenance. Mis-pointed clusters will also lose locallygenerated power. Careful consideration of the power source at least forcritical elements of the balance circuitry is necessary. It may beadvantageous to power this critical circuitry via a separatenon-volatile power source, or for the cluster voltage to be maintainedat a minimum level that allows critical operations via a low-powercentralized supply.

The power supplied to or harvested from the modules, is locally providedat least substantially from the solar cells. Excess power harvested froma receiver may be injected to the 600-800 V output of the array. For an8-receiver cluster, this involves a voltage step up of ˜8× from thereceiver voltage. This voltage step up can be made efficient byutilizing an appropriate winding ratio in the isolation transformer thatis needed to allow balance circuits to float relative to the clusteroutput. Such large voltage adjustments are generally to be made onlywhere a transformer is otherwise required, and a turns ratio is employedjudiciously to maintain efficiency.

Boost and buck or buck/boost circuitry could be used in such balancecircuits. The same voltage-ratio considerations apply for circuits thatborrow power from this cluster output and circuits that shunt powerbidirectionally. A principal advantage of utilizing locally-producedpower is a reduction of interconnect losses and costs.

The Balancers shown in FIG. 1A have inputs and outputs that areisolated. The voltage at the output is ˜N times that at the input, sinceit is the result of series connecting as many as N receivers, given thatsome receivers may be bypassed. The balancer circuit may therefore usean isolation step-up transformer having an appropriate turns ratio toensure efficient switching behavior.

The Balancers are able to detect the maximum power point of thereceiver. This generally requires the Balancer to have information aboutboth the receiver output voltage and the total receiver output current.According to the arrangement in FIG. 1A, the Balancers can readilymeasure the currents i_(ja) and voltages V_(j), but cannot as easilymeasure the common current i₀. Measuring i₀ can be accomplished byhaving a “master” controller that senses the common current and reportsit to slave controllers in each balance circuit.

Because the data is common to all controllers, it can be transmitted asa broadcast to all slaves simultaneously. By dithering or some othersuch perturbation, each balance controller can then establish themaximum power point or the path toward the maximum power point of itsreceiver.

As used herein “digital communications” may comprise conduction-basedsignaling such as differential voltage signaling (e.g., RS485, Ethernet,and others), single-ended voltage signaling (e.g., RS232), current loopsignaling (including voltage, current or others), or RF signaling thatis multiplexed on conductors (e.g., coaxial cables, twisted pairs, orpower lines), inductively coupled signals, capacitively coupled signals,free-space RF signaling (e.g., WiFi, blue tooth, or others), microwavelinks, optical signaling (e.g., via free-space, light-pipe, optocoupled,or fiber transmission and reception of LED or laser light). Opticalsignaling can involve modulated reflection or scattering of ambientlight, for example using visual patterns that are remotely discernableby the eye, camera, or other sensor.

Digital communications as used herein may be synchronous orasynchronous. Digital communications may involve 1-wire, 2-wire, 3-wire,4-wire, or more-wire signaling. These signals may utilize anapplication-specific standard or conform to established standards, suchas Ethernet, WiFi, USB, IEEE1394, modbus, CAN bus, PROFI-bus, one-wireprotocols, SPI, I2C, HDMI, or alternate standards for electronicsignaling. Some signaling standards provide acceptable communication biterror rates at the necessary data bandwidth and across the necessaryphysical distances in spite of electronic, RF, or optical interferenceat low cost. Digital signaling at the lowest level in the hierarchy maybe the most cost sensitive. Digital signaling at the highest level inthe plant hierarchy may be the most bandwidth sensitive.

FIG. 7 shows a schematic block diagram of an embodiment of amaster/slave arrangement of balancers (700). Element 702 is a mastercontroller that digitally communicates to a network of the plant (forexample a cluster-wide level in the plan hierarchy) via signals 704, anddigitally communicates to balancer slave circuits 706 via an interface708 and 710.

In the particular embodiment shown in FIG. 7, the transmissions from themaster are common (bussed) to balancer slaves, and the transmissionsfrom slaves are separate. Possible alternate arrangements may comprisebussed transmissions from slaves or separate transmissions from themaster.

The particular arrangement of the interface 708 and 710 reflects theasymmetry in communicated data. For example, a master may frequentlyneed to communicate the common string current to the balancers so thatthey can perform their power-point tracking. The master may also requeststatus, voltage, or balancer current of all devices frequently to assessthe state of the system. Communications to individual devices may alsobe required.

These different requirements can be supported by the use of a protocolhaving individual and group addressing capabilities. In someembodiments, this addressing may be separate or merged with “command” or“data identifier” bits.

Some embodiments of a master/slave balancer architecture may furthercomprise a diode 712 that blocks a reverse cluster current. However, inother embodiments such a diode may not be favored because of the forwarddiode drop voltage, large required reverse breakdown voltage and cost.

Some embodiments may further comprise a switch 714 to provide a directconnection between the cluster output and remote-conduction cable. Insome embodiments, this switch may be controlled electrically, e.g., viamaster 702. Embodiments containing 714 may be used to provide foreliminating the forward diode drop of an element 712 or may be used forsynchronous rectification.

As used herein “remote conduction” describes a plant-scale length ofconduction of power. Some embodiments may have neither 712 nor 714, andthe connection between the remotely conducted positive cluster output716, and local positive cluster voltage 718 may simply be a conductor.Such an arrangement may reduce cost and improve efficiency at theexpense of a possible increase in the requirements of the inverter tocontrol power flow to and from a cluster.

Variations are possible. Some embodiments may comprise switch 714 as amanual isolator switch to provide for servicing. Some embodiments mayprovide a bypass diode 720 external to the receiver (e.g., to provide apath for current if a collector is grossly underperforming orinoperative).

Some embodiments contain a switch 722 to provide a direct conductionpath to bypass a collector. Such a switch may further provide forshort-circuiting a receiver to facilitate maintenance, to reduce thestring voltage, etc.

Such as switch may be manual, mechanically actuated, or electricallyactuated. Mechanically actuated switches may comprise solenoid actuatedswitches such as relays or latching relays. Mechanically actuatedswitches may further comprise switches having motor or pneumaticactuation, which can provide potential benefits of large contactseparation that may be difficult or power intensive to accomplish withsolenoids.

In some embodiments switches are co-located so a reduced number ofactuators may be used. For example, a motor, such as a brushless DCmotor, stepper motor, or brushed DC motor may be used to select a switchfrom a group (e.g., 724), and another motor to actuate the selectedswitch. In some embodiments, a first motor selects a switch, a solenoidor second motor engages with the selected switch, and the first motoractuates the switch.

Embodiments using such a motorized switching array may offer one or morebenefits, including but not limited to:

-   -   high isolation voltages between actuator and both sides of        switched circuits;    -   bi-directional switch currents and voltages;    -   low switch conduction losses;    -   large contact separation distances for switching of high-voltage        circuits;    -   self-indication, e.g., the state of the switch may be discerned        by its position;    -   manual actuation as an option, and in some cases obviating        redundant switches; and/or    -   lower cost per actuated switch over individually actuated        switches. This cost tradeoff improves as the number of switches        in such a group increases.

Connection 726 represents conductors spanning a distance from a receiverto the point 728 where the receivers are physically connected in series.In some embodiments, this distance may be of the order 1 to 5 m.

Connection 730 represents conductors spanning the distance between theseries connection point 728 and the balancer input 732. In someembodiments this distance may be of the order 1 to 30 m.

Capacitor 734 may alternatively located or paralleled with a secondcapacitance at positions 728 and 732. Element 736 is a receiver from anarray of N receivers 738, that are series connected at 740.

The balancer master may measure the series cluster current, for exampleby the use of a low-side current-sensing resistor 742. Such a resistormight comprise a number of low-valued resistor technologies, includingmetal ribbon, carbon composition, metal wire, and specially-designedprinted circuit board trace.

The variation in resistance with temperature may optionally becompensated for by providing the master circuit with an ability tomeasure or infer the resistor temperature. Such a measurement may beaccomplished by thermistors, varistors, thermocouples, transistors,diodes, and/or integrated circuits, via capacitively coupled or directlycoupled known currents flowing through the same trace or a substantiallyproximal trace.

A possible advantage of the architecture is the ability for the balancermaster to measure current directly via a sensing resistor on the lowside. However, in some embodiments, the balancer master may measurecurrent via a resistor on the high side or by a non-resistor-basedmeans.

Alternatively, the current may be measured using hall effects, and othermagnetic-based measurements.

In some embodiments, a balancer master may update its currentcalibration via coordination with an inverter. An inverter may allow fora precision or precisely known current to flow to the cluster. Thiscurrent may be digitally communicated to the balancer, which couldrevise its calibration to match that from the inverter.

In some embodiments, this message and recalibration would be performedfrequently enough to obviate temperature compensation. An advantage ofsuch an arrangement is that the inverter connected to the cluster may becentrally disposed with a number of other inverters that could timeshare a common precision current sensing or reference capability tominimize the per-cluster cost of this ability.

According to certain embodiments, the balancer master may measure thecluster voltage, for example via a resistive voltage divider. In someembodiments, the balancer master may measure each receiver voltage usinga chain of voltage dividers. In particular embodiments, the master maytransmit this voltage information to a slave digitally. The balancermaster may receive reports of individual balancer voltages from itsslaves digitally, if needed.

FIG. 8 shows a schematic diagram of an embodiment of a balancer slave800 according to the present invention. In this embodiment, the balancerslave circuitry is referenced to the negative voltage from the receiver(802). In alternate embodiments, the slave circuitry could be referencedto the positive voltage from the receiver (804) to the negative orpositive voltage of the cluster, to a voltage from another receiver, toearth ground, or to another voltage. The arrangement of FIG. 8 may offera benefit in that the only necessary voltage isolation is in the digitalcommunications with the master, 806 and 808, for which a large number ofeffective and inexpensive isolator technologies are available (forexample optocouplers, inductively or capacitively coupled isolators).

Element 810 is a substantially digital controller, which may comprise anASIC, microcontroller, microprocessor, PLD, CPLD, FPGA, and the like.This controller may provide for analog-to-digital conversion of thereceiver voltage, via for example, a resistive voltage divider 812,balancer current (such as via current-sensing resistor 814), and/orother parameters, such as temperature, switch temperature, analogsignals involved with power maximization, etc. In addition, thecontroller 810 may provide one or a plurality of analog outputs (forexample 816) to a power electronics circuit 818.

One or more of these outputs could be produced in part by pulse-widthmodulation of digital signals. In some embodiments, one such analogsignal may comprise a power calculated from the product of receivervoltage and string current. In some embodiments, this signal maycomprise a power calculated from the product of the receiver voltage andsensed current. In some embodiments this signal may comprise acombination of both.

In some embodiments, the relative weighting of these combined powers maybe adjusted to account for circuit losses and sensor inaccuracy. Incertain embodiments, an analog signal is proportional to a weighted sumof the receiver and string current wherein the weights are adjustedsimilarly.

Digital control and indicator signals may pass between 810 and powerstage 818 over lines 817. Such a power electronics stage may utilizeelements of one-cycle-control or analog feedback for performing agilemaximum power-point tracking, using analog versions of signals such asthe string current or string voltage for this purpose. In someembodiments, the string voltage communicated from 810 to 818 may be thevoltage from the voltage divider 812 or a processed (e.g., filtered oramplified) version of that voltage.

According to some embodiments of the present invention, the interfacemay manipulate an analog signal to adjust the behavior of power stage818. An example of such manipulation is to adjust the control system toachieve better efficiency. In such embodiments, power stage 818 mayprovide for fast response to transients but less favorable power pointoptimization. Controller 810 may use analog signaling or manipulation oranalog signals 816 or digital signaling 817 to trim the control of 818to achieve both fast responsiveness and high accuracy.

One possible advantage offered by the particular circuit arrangement ofFIG. 8, is that analog sensing voltages, for example, those produced bycircuitry 812 and 814 may be referenced to a common negative voltage orlocal ground shared by the circuitry 818 and 810.

In some embodiments, the balancer master may assist the balancer slavewith current and voltage calibration. For example, the balancer mastermay first calibrate its own current sensing resistor, for example bydisabling all balancers and receiving an accurate current reading from acentral inverter.

Next the balancer master may enable a balancer slave to operate. Thebalancer master may then infer what actual current the balancer issupplying to the cluster by measuring the string current and receiving areport of the cluster current from the inverter.

The balancer master may then transmit this information to the balancerslave, which can use it to infer an adjustment calibration factor toapply to its own receiver-side-current sense measurement. This may bedone by tweaking an analog signal via a PWM line, by changing aparameter in the calculation of an analog signal produced via PWM orsome alternative digital-to-analog conversion. The receiver-sideparameters can thereby be adjusted relative to the cluster-side powerperformance, which implicitly accounts for balancer circuit non-idealityand inefficiency.

One possible advantage of this type of calibration approach is that itcan be designed to converge on an operating point that maximizes thepower at the central inverter without requiring high accuracy of eachbalancer measurement. Moreover, this calibration procedure may providefor better power maximization than is possible with accuratereceiver-side-only-based current measurements.

A similar technique may be used to calibrate voltage. For example, thebalancer master can short-circuit a receiver and receive an accuratevoltage report from a central inverter then re-enable the receiver andreceive a second accurate voltage report from a central inverter. Thedifference in these measurements can inform the voltage calibration ofthe balancer slave.

According to some embodiments, the balancer slave may sweep acalibration parameter over a range of values until the measured clusterpower is maximized. This procedure may be requested by the balancermaster, who would provide feedback on the results of the sweepexperiment.

In some embodiments the effect of sweeps is measured at the centralinverter. This can potentially provide better overall system poweroptimization indications than is possible local to the balancer.

In some embodiments, both types of optimizations are made at differentintervals. For example, the balancer master may undergo such acalibration at one interval utilizing the central inverter. The balancermaster may then perform a calibration of its balancer slaves at anotherinterval, either involving the central inverter or not. Calibrationsthat do not involve the central inverter may be conducted morefrequently than those that do, since the central inverter generallyservices a large plurality of clusters.

In some embodiments, a balancer master is not connected to a cooperativeinverter. In such cases, the balancer master may have sufficientcircuitry to perform power optimization without accurate quantification,because it can perform power maximization without knowing absolutepower, or it may have sufficient circuitry for absolute powermeasurement, for example if it desired to use the balancer for powerquantification.

A DC-DC power supply may draw power from the receiver to produce logicpower 822 and power for analog circuitry and switch gates, 824. In someembodiments, this supply does not allow the logic or gate power voltageto drop below a certain level, to avoid improper operation of thebalancer.

In some embodiments, this power may be provided from the clustervoltage, which may be “uninterruptible,” or some other power source.However, there may be no need to operate a balancer when insufficientpower is available from the receiver to run the balancer slave circuit.

Certain embodiments of the present invention may carefully designelements 818, 810, and 820, such that the circuits power up, operate,and power down favorably with wide swings in the power output of thereceiver.

Excess power harvested from the receiver, or the deficit power suppliedto the receiver by the balancer, depending on its configuration, comesfrom a common bus 826. In some embodiments, this bus is the clusteroutput.

In certain embodiments this bus is separate from the cluster. In someembodiments this bus is isolated, in other embodiments the bus may sharea common voltage.

According to some embodiments, the bus is serially connected to thereceiver string, for example between intermediate receivers in a string,or before or after a terminal receiver. In some embodiments, the bus isconnected to a bus voltage derived by a DC to DC converter from thecluster voltage. In some embodiments, this converter is a boostconverter, a buck, converter, or a buck-boost converter.

At an optimal common series current, the sum of the operations of thebalancing circuits is minimized. For example, if the balancing circuitswork by drawing current from the receiver, an optimal operatingcondition exists in which at least one, weakest-performing receiver isat its maximum power point with zero current drawn by its balancecircuit. Conversely, if the balance circuits supply current, an optimaloperating condition exists in which at least one, strongest-performingreceiver is at its maximum power point with zero current supplied by itsbalance circuit.

In either case, it is possible for the master to stabilize a controlloop near this optimal condition, by monitoring the presence of aresponse or flag set by one or more balance circuits that they are at orpast a zero-shunt-current maximum-power condition. If the master detectsat least one zero-current condition report, it can instruct all balancecircuits to increase their average current draw/supply over thefollowing time interval by an increment until no balance controllers aresignaling a zero-current condition. If no balance circuits are reportingzero-current, the master can instruct all slaves to reduce their averagecurrent draw/supply. By repetitively operating in such a manner, themaster can ensure that this optimum operation is closely tracked.

The master controller can alternatively force controllers to adjusttheir balance circuits in a particular direction by intentionally under-or over-reporting the common current. Of course more sophisticatedcontrol methods can be employed to improve the maximum power-pointtracking frequency response for given signaling bandwidths, or if thebalance circuits are bi-directional and the optimal operating point isnot sensed by a zero-current condition. Other considerations, such asminimum or maximum operating voltages or currents may require thecluster to operate away from these maximum-efficiency settings.

The Balancer is able to measure the receiver voltage(s) and the balancecurrent. The total power can be calculated by multiplying the measuredvoltage with the total receiver current.

Circuitry associated with the balancer can be used to provide benefitsfor the plant. One example of such a use is the actively control of themaximum cluster voltage, such as to avoid over voltage if the clusterload is low. In some embodiments, such a function could be performedrapidly via switches associated with the balancers.

According to certain embodiments, this function may be hardwaresupported. For example, a minimum number of collectors could beshort-circuited if an overvoltage threatens. This threat could bedetected by a fast comparator, allowing the balancer to assist withprotection of the cluster's inverter.

In some embodiments, output of a cluster could be clamped by aconventional over-voltage suppressor such as a zener diode. In suchcases, the suppressor reduces the need for response speed. When therequired response speed is sufficiently low, the balancermicrocontroller may be able to force a short circuit in one or morecollectors before any device is damaged.

Alternatively, the cluster's inverter can be designed such that it cantolerate the full open-circuit voltage of a high-performing cluster.Design tradeoffs in the inverter switches, however, may lead to reducedpower efficiency or increased cost.

In some embodiments the inverter is designed to handle the fullopen-circuit voltage. An active clamping scheme utilizing the balancermay provide an additional margin of safety.

The balance controller may need to convert the digitally reported commoncurrent reading it receives to an analog signal, e.g., via a pulse-widthmodulation (PWM) or digital to analog (D/A) peripheral, if the maximumpower-point optimization is done via an analog control system. Thebalance controller can intentionally skew this analog signal to forcethe control loop in a desired direction.

An advantage of using an analog control system is the ability tomaintain a tight and accurate high-frequency control loop withoutperturbations from microcontroller latency and multi-tasking. A hybridof the one-cycle control (OCC) methodology” managed and adjusted by amicrocontroller offers a good balance of frequency response andsophisticated controllability. Such a scheme is referred to as a managedOCC (mOCC).

This circuitry may require a frequency response of at least several Hzto several tens of Hz if dynamic fluctuations, e.g., from wind-loadingare to be compensated. Simple digital control schemes generally requirea high degree of oversampling to perform well. Digital loop frequenciesas high as 10 kHz are practical with modest microcontrollers linked viaan isolated communication scheme, e.g., SPI, I2C, or RS485. Analogcontrol loop frequencies can be much higher in a mOCC arrangement.

FIGS. 2A-2D show a variety of simplified balancer schematic diagramsaccording to embodiments of the present invention. FIG. 2A shows abalancer circuit arrangement (200) in which the balancer circuit foreach receiver is separate. The receiver power source equivalent circuit(202) shows interconnect inductance and possibly added inductance (204)in its path to the series connection 206. Additional interconnectinductance and possibly added inductance (208) is in the path to thebalancer circuit.

It may be desirable to design the architecture of the switching circuitto make use of this inductance, for example by placing the switchingcircuit in series with an inductor in the filter circuit 210. Element212 is drawn as an enhancement mode N-channel MOSFET, but can be anyelectrically controlled switch. The full bridge 214 drives a coil instep-up transformer 216.

Elements 218 and 219 represent the transformer leakage inductance and insome cases an added inductor. Alternately, a half-bridge driver could beemployed as is known in the art.

The full bridge 220 and filter 222 comprise the side of the balancercircuit that works at the cluster voltage (224 to 226). By the symmetryof the design, this balancer circuit can be bidirectional.

FIGS. 2AA through 2AC show simplified schematics of alternative circuitsthat provide for bi-directional power flow according to embodiments ofthe present invention. Circuit 230 in FIG. 2AA shows a bidirectionalisolated flyback converter. Depending on the switch timing, current canflow into receiver terminals 231 and out of the cluster 232 or out ofreceiver terminals 231 and into the cluster 232. The flyback step uptransformer 233 must be carefully designed with a substantial leakageinductance. In some embodiments of the present invention, the flybacktransformer 233 may comprise additional taps or windings to improveefficiency by recovering demagnetization energy.

FIG. 2AB shows a simplified schematic diagram of a half-bridgebidirectional converter 235 according the an embodiment of the presentinvention. This converter comprises two switches on both the receiverand cluster side and a discrete inductor 236. This additional circuitrymay improve conversion efficiency over that of the circuit in FIG. 2AA.

FIG. 2AC shows a simplified schematic diagram of a bi-directional Cukconverter. This converter has the advantage of fewer switches than inthe circuit of FIG. 2AB. However, the full converted power must passthrough multiple capacitors 239 and other components that are notlossless in practical implementations, which in some cases may reduceconversion efficiency.

In unidirectional operation, this bridge works as a synchronousrectifier, which could alternatively be replaced by a diode bridge.Alternatively, the bridge 214 could be replaced by a diode bridge.However, because of the lower voltage on this side of the transformer,the diode's forward voltage would have a greater effect on efficiency.

As either a diode bridge or switch bridge, element 214 can be used tobypass a receiver. However losses in the filter 210 and perhapsinductance 208 may justify placing an active or passive bypass closer tothe receiver.

Each balancer can be completely modular, or may share control circuitryto provide for enhanced coordination between balancers and reducedcomponent costs. A degree of soft switching can be achieved by judicioususe of the inductances and capacitances on the receiver side (204, 208,210, and 218), and in some cases by coordinating timing betweenswitching of the balance circuits through the series connections 206 onthe receiver side and parallel connections 228 on the cluster side.Coupling on the cluster side could be enhanced, for example by theaddition of one or more passive elements, such as inductor 230.

FIG. 2B shows an alternative arrangement or balancer circuitry 240 inwhich a single multiple-tap transformer 242 is employed. Separateswitcher circuits 244 feed isolated taps 246 of the transformer.

A common switcher circuit 248 is connected to the cluster voltages. Forunidirectional operation, this switcher circuit could be replaced by adiode bridge. However, unlike in FIG. 2A, at least one active switch isrequired in the circuit 244 to control the relative amount of balancingcurrent flow.

The use of a single transformer produces strong coupling between thebalancer circuits 244, providing for soft switching via careful timesynchronization. The use of a single transformer may provide additionalsavings in magnetics.

The transformer can be sized for the maximum imbalance power to beprocessed. This limit is generally less than the sum of the maximumpermissible power imbalances of each individual receiver.

The use of a single cluster-side circuit represents a considerablereduction in part count and hardware complexity. A degree ofsoft-switching may be accomplished through careful coordination ofswitching timing.

FIG. 2C shows a balancer circuit 260 that employs a passive rectifierbridge 262.

FIG. 2D shows a balancer circuit 270 that employs a transformer 272 thathas isolated, coupled windings 274. Rather than flowing power to or fromthe cluster output, this circuit draws and supplies currents as neededto operate each receiver near its maximum power point.

FIG. 2EA to 2EC shows simplified schematic diagrams of alternativeembodiments of unidirectional isolated converter circuits according toelements of the present invention that have switching circuitry solelyon the receiver side. In some alternative embodiments, these convertersmay be reversed such that all switching circuitry lies on the clusterside.

FIG. 2EA shows an isolated flyback converter 280. The isolation andstep-up transformer 282 is designed to have leakage inductance to permitthe low-component-count circuit. In some embodiments of the presentinvention, the transformer is designed to have additional windings ortaps that feed circuitry that recovers magnetization energy that wouldotherwise be lost when switching.

FIG. 2EB shows a schematic diagram of an embodiment of a two-switchforward isolated converter circuit 288 that may be employed in abalancer according to the present invention.

FIG. 2EC shows a schematic diagram of an embodiment of an isolated Cukconverter circuit 294 that may be employed in a balancer according tothe present invention.

The circuit embodiments in FIG. 2 may contain additional auxiliarycoupling circuitry to provide for soft switching from one phase to thenext. In some embodiments, this circuitry includes coupling capacitors.In some embodiments this circuitry includes inductively coupled windingson an isolation transformer. In some embodiments, this circuitry isconnected as a daisy chain from one receiver to the next. When aparticular balancer is not switching, it may remove itself or be removedfrom the daisy chain by the use of semiconductor or mechanical switches,such as relays or ganged, motor-actuated switches.

In some embodiments, the timing of switching or power transfer betweenreceivers is carefully coordinated. In some embodiments, thiscoordination is performed via one or more handshaking signals. In someembodiments, the balancer master coordinates the switch sequence timing.In some embodiments this timing is coordinated in part by the balancerslave circuits. In some embodiments, some timing handshaking or clocksis implemented as a bus. In some embodiments some timing handshaking isimplemented as a daisy chain. In some such embodiments, timinghandshaking signals may be bypassed by a balancer slave or balancermaster. In some cases this bypassing is performed using a semiconductorswitch or logic gate. In some other embodiments this bypassing isperformed using a mechanical switch, relay, or motor-actuated switch. Insome such embodiments, the switch may comprise several throws thatinclude all circuitry needed to soft switch to distribute power from onebalancer slave circuit to another.

The source, destination and magnitude of balance current depends on thetime sequencing of the switches. By judicious time sequencing, switchingstresses and losses can be reduced. These losses may be further reducedby the use of passive soft-switching and active soft-switchingtechniques, for example quasi-resonant and resonant switchers.

In certain embodiments, the circuits 200, 240, 260, and 270 are locatedin a physically compact space that may serve to achieve one or more ofthe following results: reduce EMI/RFI; facilitate carefultime-sequencing; maximize the use of shared components (such asprecision clocks, power supplies, communication buses, etc.); and/ormaximize the inductance 208, which supplements that of 210.

In some embodiments, a single, relatively powerful processor may performall switching timing calculations and control. In other embodiments,several less powerful processors may perform switching calculations andcontrol for individual receivers, coordinating via communications, andhandshaking.

Certain embodiments may employ OCC or mOCC in order to perform switchingcontrol. The general complexity of optimizing the bidirectionalswitching of N circuits to each of the N−1 others for maximumpower-point, best efficiency, and lowest noise/ripple, favors the use ofcomparatively sophisticated digital processing.

One objective in the design of power circuitry is the need to operatewith reasonable efficiency at relatively low currents, while retainingrelatively high current capacity. The maximum-efficiency point should bebiased below the maximum permissible power setting, and the roll off inefficiency should be optimized, for example by the use of variableswitching frequencies, pulse-skipping, soft-switching, and/or othertechniques.

These switching circuits should be carefully interleaved to reduceswitching losses and minimize the instantaneous departure from maximumpower-point operation of each receiver. Judicious interleaving canreduce the efficiency roll-off at low balance currents.

The following summarizes one or more characteristics of particularembodiments of a cluster.

-   -   A cluster comprises O (8) receivers in proximity with an        enclosure within about 8-10 m of wire of all receivers.    -   Receivers are electrically connected in series to achieve the        maximum voltage for the number of die connections.    -   Isolated switching circuits, called “balance circuits” (ideally)        draw imbalance current, supplement imbalance current or draw and        supplement (bi-directional) imbalance current.    -   The power drawn or injected by balance circuits is consumed or        produced locally. That is, power balancing is self-contained,        requiring no extra long runs of high-power-bearing cables to a        remote location to support power maximization.    -   Balance circuits are able to efficiently bypass receivers that        underperform by an excessive amount. This can be accomplished        passively by the use of a bypass diode or with a switch, and in        some cases a switch that is already used otherwise in the normal        operation of the balance circuit if series resistance of the        circuit is sufficiently low.    -   Within a cluster, all balance circuits are contained in one        enclosure that is mounted for convenient servicing. Co-location        also provides for fast communications and shared elements, e.g.,        clocks. Some elements of balance circuits must be electrically        isolated from each other.    -   The enclosure can also house the inflation and tracking        controller(s), which optimize the power distribution on the        receiver. This controller(s) can also perform role of “master        controller.”    -   Low-frequency input filter capacitors may be located here        because of their lifetime limits and possible need for        replacement. This configuration provide for easier maintenance.        Therefore, although the configuration of this embodiment may not        be as ideal as placing the capacitors directly at the receiver        because of the inductance of the intermediate wires (˜1-15 uH),        maintenance concerns may make this configuration more desirable        even though there is a potential performance penalty.    -   Water cooling is readily available in this enclosure and is used        liberally to extend the lifetime of equipment, eliminate        excessive heat sinks, and reduce size.    -   Consumable components, e.g., capacitors, IGBTs or MOSFETS and        their drivers are modular for easy and safe field replacement.        The cost of consumables and their replacement is minimized        Consumables are enclosed to prevent secondary damage on failure.

Central Inverter Design

The moderately long DC transmission lines from each cluster, or in somecases from small groups of clusters, will individually converge at acentral plant location housing a massively interleaved inverter. Eachtransmission line pair goes to a separate interleaved inverter.

Such an arrangement does not in principle add to the conductor cost overan arrangement in which conductors are merged; rather it may reduce theconductor cost by providing a convenient way to optimize the totalconductor cross-section at each point in the plant. However, thisarrangement adds to the insulator cost. This cost should be offset bymaking power balancing between phases trivial and allowing differentclusters to operate at different voltages, according to a lessconstrained overall efficiency optimization.

At some plant scale, the cost of DC interconnect wires to transmit powerefficiently from clusters to a distant inverter, may justify the cost ofadding a closer inverter and step-up transformer. Such large plants mayjustify a cellular structure.

It is generally most advantageous in cost and efficiency to use only onestage of transformer between a plant and the grid. But, in some casesutility requirements may demand a separate isolation transformer. Insuch cases, it may be best to consider breaking plants into smallerunits and distributing them geographically, providing the added value ofreducing the magnitude of the effect of weather and cloud factors ontotal power generation.

Thus the scale at which DC interconnect wires become prohibitivelyexpensive sets a natural plant size limit. This limit scales with thefourth power of the DC voltage, which only increases insulation costsproportionally (if that). Thus in some circumstances it may beadvantageous to press for higher DC bus voltages and bi-polar operationin order to scale plant power.

In certain embodiments, high DC voltages may require increaseddielectric thickness in the high-heat flux water-cooled primary heatexchanger. This enhanced thickness in turn increases the temperaturedrop in the dielectric. However, it is practical to use dielectricthicknesses large enough to stand off many kilovolts without excessivethermal resistance.

A second voltage limiter is the insulation on the isolation transformerof the Balancers. The switching-circuit capacitors and switches also setlimits on maximum voltage.

Beyond these physical limits, are relatively arbitrary (and muchstricter) limits set by agencies generically to ensure product safety.These agency restrictions may significantly affect the cost andavailability of commercial high-voltage equipment.

FIG. 9A shows a schematic diagram of an interleaved inverter system 900according to an embodiment of the present invention. This systemcomprises an inverter master 902 and a plurality of inverter slaves 904.This system may further provide an uninterruptible power supply system(UPS) 906.

The inverter master may digitally communicate at a high level in theplant network hierarchy via signals 908. In turn it may digitallycommunicate with its slaves via signals 910.

In this particular embodiment, the communication is shown as bussed anddifferential-signaling-based, such as RS485. However, embodiments of thepresent invention are not limited to this form of communication, and asdescribed earlier alternate communications schemes are possible.

In the embodiment shown, the blocks share a common “ground” voltage 912which is distinct from earth ground 914. The rectangle around theinverter 916 depicts the chassis, housing, and metal objects with whichan operator may contact. This is connected to earth ground viatechniques established by the power industry, for example by conductorsdriven into the earth. Other features of the inverter system that may beearth grounded include the cooling system, coolant, and other elements.

In the particular embodiment shown, the sole physical earth groundconnection referenced by the circuitry occurs at the UPS 906. Thisearthing arrangement may avoid issues associated with earth groundcurrents.

In some embodiments the central inverter establishes the cluster busvoltage reference. In the embodiment shown, the negative voltage of eachcluster is tied (e.g. 918) to a common voltage at the inverter. Thiscommon voltage is “ground”, but not necessarily “earth ground.” Theclusters may separately earth ground their chassis, support structure,and cooling system, etc., but may not otherwise tie a voltage in theircluster to “earth ground.”

In some embodiments of the present invention, the positive voltage ofclusters are connected into a common positive voltage. In somealternative embodiments, one voltage from a plurality of clusters isconnected. In some alternative embodiments, one voltage from a pluralityof clusters is connected and the other voltage from a differentplurality of clusters is connected in common.

In some embodiments of the present invention, the voltages of at leastone cluster is not connected with any other cluster, e.g., all clustersoutputs are maintained separate. In some such embodiments, the inverterslaves share a negative or positive common voltage that is substantiallyisolated from the cluster voltage. In some embodiments, this commonvoltage is constrained to fall within a range relative to one or both ofthe cluster voltages, for example, to ensure that circuitry is properlybiased.

One reason for isolating a cluster voltage from another is to allow theinverter slave to switch using a more symmetrical high-side and low-sideprofile than may be possible if a number of clusters having differentcluster voltages have one voltage in common. For example, if one clusteris producing at one voltage, another cluster is producing at a differentvoltage, a terminal of the clusters is connected together, and thecircuits are switching into the same voltage, the switch timing on thehigh-side and low-side of the two clusters may be unsymmetrical. Such alack of symmetry may increase switching, conduction, and/or core lossesover those of a symmetrical profile. In principle, a slave inverteroperating on an isolated cluster voltage, may attain a more symmetricalswitching profile and may achieve a higher inversion efficiency. Thispossible increase in efficiency may come at the cost of isolatedswitches and additional isolated power supplies. In some embodiments,the extra cost and power required for these isolated elements isjustified by efficiency gains. In other embodiments, e.g., in theembodiment shown it is not. In embodiments having isolated clustervoltages, some of the inverter-slave circuitry may share a commonvoltage to provide for simpler interfacing, e.g., of communicationlines, clock lines, hand shaking lines, and the like.

The arrangement of establishing a common voltage reference for eachcluster physically proximate to (or within) the inverter, may offer abenefit in that each inverter slave can share a ground with each otherand with respect to the master. This potentially obviates the need for alarge number of isolated power supplies, isolated communications, etc.

The potential success of such a grounding scheme may depend uponsupplying a sufficiently low impedance and inductance ground plane, suchthat ground bounce and voltage offsets produced by ground currents donot disrupt communications and other circuit functions. A physicallycompact inverter, good circuit layout, and thick ground planes/tracesmay allow individual inverter circuitry to operate without isolation.

It may also be favorable to communicate between inverter slaves and theinverter master via differential signals that can swing beyond the powerand ground rail (e.g., RS485). Analog circuitry that is over-the-railstolerant may also be favorable.

According to this particular embodiment, the master inverter containsfilter capacitors 920. The inverter slaves in this embodiment eachfunction substantially as current sources, and so may share a commoncapacitor.

The individual filter inductors are contained in the inverter slaveblocks 904. By interleaving the switching times of each inverter slave,the necessary size of this filter capacitor may be reduced by a largefactor. In some embodiments, the inverter slaves may functionsubstantially as voltage sources and share a series filter inductorwhose inductance may be similarly reduced.

To reduce cost and provide for tightly synchronized operation, themaster may supply a clock signal 922 to inverter slaves. In certainembodiments, this clock signal may be from a microcontroller.

Improved interleaving may rely upon accurate temporal switchingcoordination. Accordingly, the master may produce, listen, and/orcontribute to a handshaking bus of digital timing signals (924) utilizedby the inverter slaves in part for establishing switch timing andsynchronization.

In the particular embodiment shown, the logical voltage 926 andgate/analog voltages 928 may be supplied by the uninterruptible powersupply 906 and are shared for inverter slaves. To avoid unwantedpower-supply inductance, these power supplies, while fed by the UPS maybe placed proximate to or within the inverter.

In certain embodiments, the UPS provides an intermediate voltage, andeach inverter slave individually produces its operating voltages fromthis source. Such an arrangement may be favorable if the power-supplyinductance is large, or extra voltage isolation is desired betweeninverter phases.

According to some embodiments, each inverter slave may have a simplesnubber and capacitor to eliminate power supply voltage excursionswithout resorting to separate supplies. Each inverter phase may alsocontain a “test port” containing signals and connections that arenormally open in operation, but that support automated testing andtroubleshooting when an inverter experiences a fault.

FIG. 9B shows an embodiment of an inverter system 950 that provides forswitch synchronization daisy-chaining via circuitry 952. Soft-switchingand high-precision switch interleaving between separate inverter slavesmay utilize high-precision daisy-chain handshaking.

For example, a slave (e.g., 954) may await a signal from another slave(e.g. 956) previous in the timing chain on one handshaking line 958,before initiating a switch sequence. At some point in this switchsequence, it may assert a signal on a handshaking line 960 to the nextslave in the chain, so that the next inverter's switching achieves asoft-switched, low-stress, or low-noise condition.

Maintaining a complete daisy chain may present a challenge when anindividual inverter slave or its associated cluster, may be inoperative.Because soft-switching may be desirable, but is not necessary foroperation, inverter slaves may be coded and designed to operate hardswitched or with reduced interleaving performance if the daisy chain isbroken.

Certain embodiments (for example that shown in FIG. 9B) may bypass oneor more inverter slave timing signals in a daisy chain, by the use of aswitch 962. Such a switch may be electronic, mechanical, etc.

If the signaling electronics are low voltage and share a common groundreference it may be favorable to perform this switching via logicdevices or analog switches. Potentially desirable characteristics ofsuch switches include low cost, low power requirements, and lowtransit-time delay. Such a switch may be thrown mechanically orprogrammatically when an inverter slave is removed from service.

Removing an inverter slave may involve physically removing the device.Thus, this function may be best handled by the inverter master or on amotherboard that remains in service when inverter slaves are removed.

FIG. 10A shows a schematic diagram of an embodiment of an inverter slave(1000) according to the present invention. This inverter slave comprisesa controller 1002 that may perform one or more of the followingfunctions: communicate with its master digitally via 1004; coordinateswitching time via handshake signals 1006; derive a processor clock from1008; and/or obtain logic power 1010 and gate-drive/analog power 1012,and a common voltage reference 1014 (ground) from a mother board. Thecontroller may control switches in a one, two, three, or more phaseinverter.

The particular embodiment shown includes a simplified schematic diagramof a three-phase buck inverter, 1016. Alternatively, an inverter slavemay comprise a boost inverter.

The particular embodiment shown comprises a boost DC-DC converter. Theseconverters utilize high-side switch modules 1020 and low-side switchmodules 1022. The buck inverter utilizes series inductors 1024, sharinga common filter capacitor between slaves off out of the unit.Alternatively, a boost inverter may comprise an individual shunt filtercapacitor and shared series inductor.

The boost circuit 1018 shown in FIG. 10A may be operated to provide anextended range of operation of the circuit. If the cluster voltage atmaximum power point drops below the threshold needed to perform aninversion to a desired voltage, the inverter may:

1) reduce its power draw until the cluster voltage is sufficiently highto invert;

2) require other inverter slaves that are connected to better-performing(higher-voltage) clusters in the system to compensate by producingdisproportionately more power at voltages where the underperforminginverter is not able to source; or

3) turn off.

One design approach ensures that the cluster bus voltage is normally inexcess of the necessary voltage for buck inversion. This approach mayemploy over specification of switch voltages. This approach may affectaverage efficiency, since the buck inverter may operate less efficientlywith a substantial voltage buck.

Adding a boost stage 1018 may provide the ability for an inverter toboost its voltage to a minimum value needed for inversion, allowing theuse of a lower peak-production cluster voltage. The boost phase may usean extra switch 1026, diode or synchronous rectifier 1028, and inductor1030 (in series with the interconnect inductance).

The inductor resistance and diode's forward voltage drop and resistance,may reduce the inverter efficiency when the boost is not needed. Undersuch circumstances it may be desirable to close a bypass switch 1032.

This bypass switch may be electronically controlled via 1034. Thisbypass switch may be mechanical switch, relay, latching relay, and thelike.

Potential benefits of switching mechanical switches using motors(discussed above in the context of the balancer circuit), may apply evenmore favorably for this kind of switch. For example, this boost/bypassmode switch would not necessarily need to be activated more than a fewtimes a day. If activated for 10 hours a day, saving 1 Volt of loss at10 A, such a switch would save $1 in electricity at $0.10/kW-hr in 100days. A sufficiently inexpensive mechanical switch activated by a motor(e.g., a simple x-y traverse robot) could cost about $1 each, providingan approximately three month marginal return on investment per switch.

By contrast, alternative switches (such as solid-state relays) typicallycost 20 to 50 times as much. And while such alternative switches mayoffer the advantage of increased switching speed, this may not be asimportant a consideration for an infrequently activated mode switch.

In some circumstances (for example at night) a cluster receives a smallamount of housekeeping power. In some embodiments this housekeepingpower may be provided over the cluster voltage lines by the inverter.

When the grid is connected, this house-keeping power may be suppliedthrough the switches 1020 if the diode 1028 is shunted, for example viaswitch 1032. However, if there is a grid outage when the cluster is notproducing, there is the possibility of a power outage at the cluster.This condition could be problematic if the cluster is not producingbecause it is mis-pointed during the day, leaving the cooling system andtracking system inoperative.

Accordingly, in order to provide a reliable, uninterruptible supply ofhousekeeping power in all conditions, a UPS such as 906 may be used tosupply power 1036 via diode 1038. This circuit sets the minimum clusterbus differential voltage. Alternatively, this supply could be appliedvia a switch.

It may be desirable to maintain this supply availability whether aninverter slave is operating or not. In some embodiments, diode 1038resides on circuitry that is not removed when servicing an inverterslave (for example on a mother board that inverter slaves plug to).

Switch 1040 may provide for an array to provide power to the UPS. Thispower could be used by other clusters, and/or could be used to charge abattery. If present, such a switch may be manual or motor actuated asdescribed earlier.

If the uninterruptible power is diode 1038 connected, it resides on thecluster side of inductor 1030 or the diode 1038 will be forward biasedwhen the boost switch 1026 activates. Because the remote conduction lineinductance in series with 1030 may be substantial, the uninterruptiblevoltage may be sufficiently low compared to normal operating busvoltages, such that the diode does not become forward biased in normaloperation. Alternatively, the diode could be switched open when theboost circuit operates.

A potential advantage of the common ground voltage of the cluster andcontroller 1014, is that current sensing 1042, bus-voltage sensing 1044,and phase voltage sensing 1046 may be performed accurately andinexpensively. Good attention to analog grounding 1048 should befollowed.

A test connector 1050 may facilitate automated testing and troubleshooting.

The inverter may accurately track the maximum power point by having asignal or calculated value that is monotonic in output power.Nevertheless, it may be desirable for an inverter to be able to reportits production power accurately.

In such cases, the current and voltage sensing of the inverter mayrequire calibration. In some preferred embodiments, a slave will compareits measurements to precision measurements reported from its master toupdate its calibration. In turn, the slave may provide similarcalibration services to balancer circuitry on the slave's cluster.

According to certain embodiments, switch timing is set using one-cyclecontrol, analog feedback. In some embodiments, maximum power tracking isperformed using one-cycle control, analog feedback.

In some embodiments the performance of the one-cycle control may betrimmed and optimized by a microcontroller. In some embodiments, theswitch timing may be calculated using one or a plurality of digitalsignal processors or suitably fast digital controllers.

FIG. 10B shows a diagram of an embodiment of an inverter slave 1080 thatuses a daisy chain of handshake signals 1082 and 1084 to perform softswitching, reduced-stress switching, low-noise, switching, or otherswitching enhancements. This is done via switch timing circuitry, and insome embodiments by auxiliary switching circuitry 1086.

FIG. 11A shows an embodiment of a UPS 1100 associated with invertersaccording to the present invention. A UPS controller 1102 digitallycommunicates with the plant via connections 1104.

In this particular embodiment, the communication is shared with theinverter slaves. In some embodiments, the UPS may communicate at adifferent level in the plant network hierarchy.

This controller performs power management from the grid 1106. In someembodiments the controller performs power management from a generator1108 such as a diesel generator.

In this embodiment, the controller maintains a battery 1110 at a goodstate of charge, and supplies an uninterruptible bus voltage at thebattery voltage. This embodiment utilizes the battery to provide alow-impedance voltage reference with respect to earth ground 1112.

In this embodiment, this is where earth ground is referenced on thelow-voltage side of the step-up three-phase transformer. The connectionbetween earth ground and the common, negative-most voltage reference inthe plant is made at some position in the battery stack (e.g., 1114,1116, or 1118) depending on what negative voltage offset is desired inthe common cluster voltage.

Variations are possible. A similar scheme could be used to establish afixed positive reference. An active switching scheme could be used toestablish an arbitrary static or dynamic relationship between earthground and cluster voltages. The cluster common voltage may be directlytied to earth ground.

During a grid outage, the controller may start generator 1108 to preventbatteries 1110 from depleting their charge. The size of batteries 1110may be small enough to provide a brief period to start up the generatoror to facilitate an orderly safe plant shut down.

In some embodiments, the UPS controller also produces logic 1120 andgate/analog 1122 voltages. The UPS system may be physically separate.For example, a generator 1108 may need to be mounted externally.

Batteries 1110 may be housed where corrosive and explosive gases cannotcause problems. The supplies for 1120 and 1122 may be proximate to theinverter slaves to minimize inductive spikes on the voltage supply.

FIG. 11B shows a schematic diagram of an electrical circuit 1150 to thestep up transformers 1152 that transform plant voltages 1154 to gridvoltages 1156. In this embodiment, transformers 1152 are connected “Wye”to “Wye.” It may be desirable to connect the plant side of the step uptransform in a “Wye” configuration as shown in FIG. 11B to limit thesecondary damage that can be caused by the failure of a power switch.For example, the circuit inside 1158 depicts a failed, fused (1160)power switch 1162 on the low-side of a buck inverter phase. Some switchfailures result in the switch having a low-resistance, presenting ashort-circuit state. In this embodiment, the resulting high faultcurrent 1164 flowing through the failed circuit may be prevented fromsubstantially affecting the common negative voltage (1166) by conductinga current 1168 through the plant battery 1110.

Without a voltage clamp circuit 1150 capable of handling this largecurrent, the failed switch might cause the voltage 1166 to swing abovethe voltage of another phase, causing high heating in body orfree-wheeling diodes of other low-side switches and possibly producingadditional damage before the fuse 1160 can blow.

FIG. 11C shows an alternative circuit 1180 for ensuring that thenegative common voltage 1182 never rises substantially above thethree-phase plant voltages 1184 by the use of diodes 1186. These diodesmust be able to survive very large surge currents. Because 1182 and 1184are common among all inverter slaves, this protection circuit may appearonce in an inverter rather than separately for each inverter slave,e.g., in the UPS module or inverter master.

In some embodiments, such as circuit may establish a common negativevoltage without a battery or independently from a battery.

Alternatively, if the positive voltage of all slaves are common, thispositive voltage may be prevented from swinging substantially belowother phases by a single copy per inverter system of the circuit as1180, with the diode directions reversed.

In some embodiments, such a circuit may establish a common positivevoltage without a battery or independently from a battery.

Protection circuitry such as 1180 may be applied individually perinverter slave to the non-commoned cluster voltage, that is, thepositive cluster voltage if the negative cluster voltage is commoned andvice versa. Because of the need for the protection diodes to survivelarge fault currents such diodes may be relatively expensive. In someembodiments, this individual protection may not be cost justified andthe designer may choose to allow two or three same-side switches to failin tandem rather than to pay the expense of the individual protectiondiodes.

According to certain embodiments of the present invention, the switchmodules shown in FIGS. 10A and 10B may be engineered to be easilyreplaceable and to minimize secondary damage caused by a switch failure.FIG. 12A shows a schematic diagram of such a low-side switch module 1200comprising a low side driver module 1202 and a switch module 1204.

The switch module depicts an IGBT and free-wheeling diode, but this isnot required by the present invention. In other embodiments a switchmodule could comprise other elements such as a power MOSFET, solid-staterelay, and/or other switches.

Power switch failure may result in a short circuit across the gate. Sucha short circuit can expose the driver to excessive voltage and current,and result in destruction of the driver circuit 1208.

Embodiments of a switch module may prevent or reduce this damage by theuse of a over-voltage suppressor 1210. Such a suppressor may comprise azener diode and a gate resistor 1212 and in some embodiments a fuse1214.

According to some embodiments of the present invention, the gateresistor may be sized to burn out in an over current situation andprovide the fusing function. Such a resistor may also be favorable foravoiding latch up in some IGBTs. The circuit protection element 1210 maybe sized to survive without damage long enough for the gate fuse orresistor to open its circuit or reach a sufficiently high impedance thatthe protection element can withstand the gate current.

In spite of protection, a gate driver module 1202 may still besusceptible to damage and may therefore be packaged as an easilyreplaced, cost optimized consumable.

In some switch failures, the switched terminals 1216 and 1218 become ashort circuit. This failure may generally lead to secondary failuresrelated to over current, and could result in significant additionaldamage from excessive power dissipation.

For this reason, the power module is fused. In some embodiments, thefuse is between the switch and the terminal 1218. This fuse may comprisea conventional fuse, a feature on a PCB trace that breaks or becomeshigh impedance, a thin or thick film device, a wire, or other forms offuses that are known.

FIG. 12B shows an embodiment of a high-side switch module 1250. Thismodule is similar to that in FIG. 12A, but contains a high-side switchmodule 1252 that provides boot-strapping capability via elements 1254and a digital isolator 1256. In some embodiments in which bootstrappingis not an option (e.g. static, high-side switches), a high-side drivermodule may alternatively contain an isolated gate drive power supply.

As with the low-side driver module, the high-side driver module andswitch module may be designed such that failure of the switch does notnormally produce a driver failure, but that the driver module is acost-optimized, easily-replaced consumable.

In some embodiments, switch modules may be packaged as a cost-optimizedand easily-replaced consumable or reparable module that neverthelesspossess exceptionally good heat transfer characteristics. FIG. 13A showssuch a switch module 1300.

Element 1302 is a high-thermal conductivity base plate. In someembodiments this plate is constructed from aluminum or copper. In otherembodiments this plate may be another metal or a ceramic material.

In some embodiments, this plate is covered when installed with thermaltransfer grease or a high-heat transfer compliant material such as a“sil pad”, to reduce the contact resistance between the plate andexternal heat collector. According to certain embodiments of the presentinvention, this element 1302 is insulated electrically from the switch.

Element 1304 is a module cover. In some embodiments this module cover ismolded or cast from a high-temperature-tolerant material, for example athermoset resin such as epoxy or phenolic, or a suitablyhigh-temperature thermoplastic such as polyester or polyamide, with orwithout fibers or particulate filling. Desirable properties for thismaterial include high-temperature stability, flame resistance, impactresistance, strength, and insulation.

This module cover may be relied upon to resist forces from acatastrophic failure of the switch and fuses contained within, which maybe accompanied by an over pressure and spray of hot metal and momentaryarcing. The cover contains physical effects of such failures, avoidingdamage to equipment outside the cover. Elements 1306, 1308, and 1310 areports of the switch (e.g., the gate, collector, and emitter,respectively).

FIG. 13B shows the interior 1320 of the switch module 1300. In thisembodiment, heat transfer plate 1302 is electrically insulated from aheat spreader 1322 by a thin insulator material 1324. This material maybe any suitable, high-temperature, high-thermal-conductivity electricalinsulator, such as mica and synthetic mica plates, alumina ceramics,polyester, polyamide, or polyimide film, etc.

To reduce thermal contact resistance between the electrical insulatorand the heat transfer plate, one may employ a mechanical preload, e.g.,via screws 1326. Alternatively or in addition, thermal contactresistance may be reduced by the use of heat transfer greases,adhesives, or other known techniques.

The heat spreader 1322 may be constructed by a high-thermal-conductivitymaterial such as aluminum or copper or alternatively another metal oralloy. This element may be designed to reduce the heat flux needed topass through the electrical insulator by a factor of 2 to 10 to reducethe effective thermal resistance of the electrical insulator.

This reduction may be accomplished by making the heat spreader thick.For example the heat spreader may be on the order of the size of the dieor thicker and correspondingly wide at the base, such that the anglefrom the edge of the die to the corner of the base of the spreader is aminimum of approximately 45 degrees, as shown.

In certain embodiments, the side walls of the heat spreader may notdisplay such a taper. The base size could have approximately the samecross-sectional area and relative relation to the die as shown.

In this embodiment, element 1328 is a fast-recovery Schottky“free-wheeling” diode die. In this embodiments, elements 1330 areparalleled IGBT die. These die are attached to the heat spreader via aconductor using techniques and bonding materials known in the art, suchas solder, silver-filled epoxy, and the like.

Elements 1332 are wire bonds. Examples of wire-bond materials includebut are not limited to aluminum and copper. Other wire materials mayinclude gold.

These wires connect the die to pads on circuit board 1334. In thisembodiment, element 1336 is a gate resistor (such as a thin-film 0805,0604, 0402 or other resistor) designed to fail open or as a highresistance during a sustained high-current pulse. In alternativeembodiments, this resister may be a thick film, a printed resistor oralternate resistor. If a resistor does not have adequate fusingproperties, it may be series connected with a fuse.

Element 1338 is a circuit trace designed to fuse when an excessiveemitter current is encountered. This trace is designed not to produceexcessive resistive losses in normal operation, but to fail quickly whena switch operates far above its rated current, indicating a fault.

Elements 1340 are mechanical supports for the printed circuit board. Insome embodiments, these supports are rivets pressed into the heatspreader 1322 that conduct current from the back metallization of thedie through the heat spreader to the printed circuit board and then outof the module via a connector (e.g., 1308).

FIG. 13C shows a view of the base (1350) side of the switch module cover1304, showing a feature of the cover designed to mitigate externaldamage in the event of a failure. In particular, during a failure hotmetal, particulates, and arcs can spray inside the module cover. Thesecan lead to a dangerous over pressure.

To avert an overpressure and control the location and rate of release ofexcess pressure from the module without releasing harmful particles tosensitive circuit areas, vents and/or filters can be used. These ventsand/or filters can be combined with the structure of the housing.

In FIG. 13C, element 1352 is notch cast or molded into the cover base1350 that serves as a vent hole. During a failure, particles and gasesrapidly enter this channel. Some particles are unable to turn the cornerinto channel 1356 and are trapped in reservoir 1354. The remainder ofparticles and gas pass through 1356.

Some particles are trapped in secondary reservoir 1358, having beeninertially separated from the gas by the abrupt turn into channel 1360.Finally some particles are trapped in tertiary reservoir 1362.

The gas, now cooled by passage and stripped of most of its particles andall of the liquid metal particles escapes through external vent 1364. Aduplicate vent 1366 may provide for reduced back pressure.

The necessary area and number of the of vents depends on the expectedamount of energy release during a failure and air volume trapped withinthe cover. The ports for terminals 1368 may be designed to resistventing, since in some embodiments, venting through those ports may putparticles in sensitive areas.

Variations are possible. In some embodiments, venting and filtering mayemploy mechanical filters instead of cyclonic or inertial filter. Insome embodiments the cover may be designed with multiple walls tocontain particles or with a compressible element to contain expandinggasses without leaking.

FIG. 14A shows a simplified mechanical drawing of the assembly 1400 ofswitch modules 1402, low-side modules 1404, and high-side driver modules1406 into an inverter slave housing 1401. In the particular embodimentshown, the low-side and high-side driver modules contain a feature 1408that indicates insertion direction and facilitates module removal. Therecesses 1410 and 1412 in the surface of 1401 establishes the moduleinsertion orientation.

In some embodiments, the physical design of the module makes itimpossible to insert in the incorrect orientation or slot, e.g., theshape of modules 1404 and 1406 are designed not to fit into each other'sslots. Switch modules 1402 may be similarly keyed to avoid incorrectorientation and to avoid plugging an incorrect module, e.g., wrongvoltage or current rating, switch type, etc. into a module recess, e.g.,1414.

In this particular embodiment, the ledge 1416 presses against the modulecover. This provides mechanical decoupling of forces normal to the heattransfer plate 1302 and electrical terminals, e.g., 1308. This pressurecan also provide an enhanced seal that prevents particles from enteringthe inverter slave, since gases and particles are vented between theplate 1302 and cover base 1350.

FIG. 14B shows an arrangement 1440 of low-side 1404 and high-side 1406driver modules and switch modules 1402 inserted into an inverter slavehousing 1401, according to an embodiment.

FIG. 14C shows a simplified internal assembly 1460 of an embodiment ofinverter slave. This figure illustrates one manner, in accordance withone embodiment, of orientation of modules 1402, 1404, and 1406 withrespect to the printed circuit board 1462.

Elements 1464, 1466, and 1468 are indicators, such as light emittingdiodes (LEDs). In some embodiments these indicators provide simplefeedback about the state of the inverter. For example, these indicatorscan indicate:

-   -   all indicators off=no power (inverter slave disconnected from        mother board);    -   indicator 1464 glowing or flashing red=fault, require service;    -   indicator 1466 glowing green or yellow=boost circuit activated,        indicating that it is not safe to close boost-circuit bypass        switch (e.g. 1472);    -   indicator 1468 glowing green=operating or passed self check:        ready to engage. Alternatively indicators could be multiple        color LEDs.

One possible goal of embodiments according to the present invention, maybe to allow for convenient maintenance. Accordingly, simplifying thehuman interface may be important.

For example, it may be undesirable to use a red LED to indicate anythingbut a fault. On the other hand, it may be important to use flashing LEDsto indicate faults so that color-blind operators are not at adisadvantage. In some embodiments, flashing and red light is used as afault indication and steady and green light is used as a normaloperation indication.

Elements 1470, 1472, and 1474 are switches that may be used to optimizeinverter efficiency, such as bypass the boost circuitry when it is notneeded. In some embodiments there may be no switches, one switch, twoswitches, three switches or more according, to how many optimizations oroperating modes are available. These switches can be actuated manuallyor via a robotic traverse, and can provide a clear visible indication ofswitch state.

Elements 1476 and 1478 are power inductors. For example, element 1476may be the inductor for a boost circuit and 1478 may be an inductor in abuck inverter phase. These inductors may generate significant heat andare mounted in good thermal communication with the top of the slaveinverter housing 1401, where active cooling may be applied.

FIG. 15A shows a top view of an inverter slave 1500, and FIG. 15B showsa back side view of an inverter slave 1500, in accordance with anembodiment of the present invention. Elements 1502 are rack slides whichmay be used to guide the inverter into place in a rack, and to transfercompressive preloads on heat transfer elements to inverter structure.

Element 1504 is a raised section of the inverter housing. Element 1504may provide good thermal contact with an active cooling plate totransfer heat from a power inductor.

Element 1506 is an element of the inverter that makes early contact withthe mother board. Element 1508 are the high-voltage power connectors.Element 1510 is a lever used to engage and disengage the inverter slave.Element 1510 may alternatively be another physical mechanism thatprovides mechanical advantage, such as one or a plurality of levershaving different characteristics or orientation from that in 1510, oneor a plurality of screws, or a combination of different mechanisms thatachieve substantially the same function as element 1510.

According to certain embodiments, the invert connectors and electronicsmay be engineered so that the inverters can be “hot plugged,” in someembodiments via one or more of the following operations.

1) Inserting the inverter phase to the rack causes the inverter housingto be a earth ground and drains off electrostatic charge to groundsafely.

2) Sliding the inverter to the end of its travel with the lever 1510 inthe up position, stops the inverter slave short of any electricalconnections other than earth ground.

3) Lowering the lever 1510 to the down position allows the inverterslave to be pushed further, allowing the logic circuitry to engage withthe motherboard but preventing any high-voltage power connections fromcontacting.

4) The order of circuit contacting may be: inverter slave ground (1014),then logic power (1012), then gate power (1010), then processor clock(1008), then communications and handshaking circuits. In someembodiments, the clock signal may be applied before the gate powercircuit.5) In this lever position, the unit undergoes a boot up self check andcommunicates with the master inverter. If the unit checks out, it turnson a green light, such as a green, slowly flashing light, to indicate tothe operator that it is ready to be engaged with high-voltage power. Ifnot, the unit turns on a fast flashing red light to indicate that theinverter is not ready to be engaged.6) If the unit indicates a fault, the unit is removed and taken to atest fixture for automated diagnosis and troubleshooting. When the unitis repaired, the fixture may set a flag indicating a clear state ofhealth. In general, a fixture may set a usage timer that may expireperiodically to prompt an inverter for return to the test fixture forpreventive maintenance, diagnostics, and recalibration.7) If the unit does not indicate a fault, the user then engages thehigh-voltage power electronics by pulling up on lever. This levercontains a cam which forces the inverter backward with mechanicaladvantage.8) The unit detects the state of the power and begins to engage inswitch synchronization and power inversion.

To remove a unit the following procedure may be employed:

1) One may choose to do a “safe shut down” by alerting the mastercontroller of this intention via a software interface. This may in turnalert the cluster to go into a non-power producing mode, tell theinverter to stop switching, etc.

2) The user lifts the engagement lever 1510, removing the high-voltagepower connections.

3) The user then pulls the unit out of its slot for maintenance or pullsthe unit out far enough to disengage logic power, raise lever 1510 andslide the inverter slave back to rest in the full disengaged position.

In some embodiments of the present invention, an inverter will be housedwith a supply of spare consumables, a test fixture, and/or a number ofoperable spare inverter slaves, so that down time for an inverter phaseis limited to the time required to swap out inverter slaves. Amaintenance worker can replace consumables and use the test fixture tore-commission inverter phases offline.

FIGS. 16A and 16B respectively show isometric top front and back views1600 of an embodiment of an inverter slave engaged with its activecooling module 1602. The inverter housing top surface has an wedge angle1601 such that as it slides toward the back of its travel it engageswith the bottom surface or cooling plate 1603 of the cooling unit 1602.

Element 1604 is a coolant inlet port, and element 1606 is a coolant exitport. Elements 1605 and 1607 are coolant bypass ports, allowing thesecooling heads to be arrayed in parallel.

Elements 1608 are reinforced rack rails (reinforcements not shown) thatmaintain a large mechanical preload between the elements 1504 and 1402and the cooling module bottom surface or cooling plate 1603. Otherreinforcements between the cooling module and rack, which do not movewith respect to each other, are also not shown for clarity.

Raising the lever 1510 causes a cam to bear on rack-mounted pin 1610with mechanical advantage that slides the inverter slave toward theback. Working with the wedge angle of the inverter top in this mannerproduces a significant mechanical preload to reduce thermal contactresistance to cooling module.

FIGS. 17A-17D show components of the coolant module embodiment 1700. InFIG. 17A, element 1702 is the coolant module housing which provides areinforced back wall of the coolant flow conduit and provides for fluidentry and exit through engineered, leak proof connectors employingredundant o-ring seals in a male connector 1704, which mates with anengineered female connector 1706 of another coolant module 1700.

A spring clip 1708 holds the mated connector securely. The coolantmodule housing seals to the cooling plate via an o-ring that ismaintained under compression by near continuous spring clips.

FIG. 17B shows the coolant module with the module housing removed,revealing the flow splitter plate 1710. The flow splitter plate containsarrays of nozzle orifices that direct coolant from the cool side (shown)to the hot side (obscured) by the splitter plate. By inertiallytransporting cool coolant to the coolant plate, these jets efficientlycool against high heat flux with low flow pressure drop.

This splitter plate contains features (such as 1712 and 1714) to make asubstantially leak-free seal from one side of the splitter plate to theother via the coolant module housing and other features 1716. This makesa substantially leak-free seal from one side of the plate to the othervia the cooling plate. Leaks from one side to the other of the splitterplate do not result in external coolant leaks.

FIG. 17C shows the cooling module 1700 with the housing and splitterplate and o-rings removed, leaving the cooling plate 1603 and end clips1720 remaining. The cooling plate contains clip features 1722 to retainthe housing against the internal pressure. The end clips provide thisretaining feature (1724) over the region that the cooling plate does nothave this feature.

FIG. 17D shows the full cooling module 1700 with the location of thecooled components, 1300, 1478, 1476, etc. relative to the cooling platerevealed. Heat from a switch flows from die 1330 through spreader 1322through electrical insulator 1324, through plate 1302 through coolingplate 1603 and into the coolant.

FIG. 17E shows an alternate embodiment of the interface 1780 between thecooling plate 1782 and a cooled component, e.g., a switch module havingcooled plate 1784. Nesting, and in some embodiments intentionallyinterfering and binding features 1785 and 1786, reduce contactresistance by increasing the total mechanical preload forces between thesurfaces for a given normal force applied between the elements 1788.This reduced force may reduce the mechanical reinforcement required bythe frame and the loads transferred through the cooling system plate tothe cooling system housing for a given thermal resistance. The inclinedforce 1789 applied, e.g., by a front-panel lever, may further reduce themechanical effort required of an operator to obtain a high-conductivitythermal interface.

FIG. 17F shows an expanded edge view 1790 of the cooling plate 1782 andcooled plate 1784 interface. Because of the wedge geometry a normalpreload force 1791 produces much larger opposing preload forces 1792that are largely borne by stresses internal to the plates, e.g., at1793. This technique is mechanically efficient because it creates andresolves the large forces locally, using internal stresses rather thanrequiring physically large and remote structural elements to bear highloads. This arrangement may further minimize detrimental thermalexpansion effects on the mechanical preload 1793. In some embodimentsthis interface is designed to increase the mechanical preload underconditions of relatively high cooled-plate temperatures so that thermalexpansion provides a beneficial effect.

FIG. 17G shows and alternative embodiment of a cooling plate design 1765that may further reduce the loads and sensitivities to compliancebetween the cooling system and support rack and inverter slave housingand support rack. In this embodiment one or a plurality of features,e.g., 1796 may mechanically interlock with mating features on a cooledplate such that the interlocking features bear a substantial portion ofthe normal force between the cooled and cooling plate. These featuresmay be designed to allow multiple cooled plates to be interlocked bystaggering the heights between successive patterns as indicated by 1797.

FIG. 18A illustrates the cold side of the splitter plate 1710. FIG. 18Bis an enlarged view of the cold side of the splitter plate 1710illustrated in FIG. 18A. Within the splitter plate, there arearrangements of features 1802 designed to cool a switch moduleefficiently and features 1804 to cool an inductor efficiently. Theactive splitter elements include nozzles (e.g. 1806) and support posts(e.g. 1808). The support posts transfer preload forces to maintain lowthermal contact resistance.

FIG. 18C illustrates the same patterns 1802 and 1804 on the hot side ofthe splitter plate. FIG. 18D is an enlarged view of the hot side of thesplitter plate illustrated in FIG. 18C. The rear surfaces of the nozzle(e.g. 1810) and spacer (e.g., 1812) are indicated. The spacers protrudefurther than the nozzles and establish a nozzle stand-off distance formaximum jet-cooling efficiency.

In some alternative embodiments of the present invention, ridges andvalleys or other features on the cooling plate may alternatively set thenozzle stand-off distance. In some embodiments, structure of the nozzlewall may bear on features on the cooling plate, performing a combinedfunction of establishing the jet and the spacing of the jet on thesurface. In some embodiments, a nozzle sprays a jet into one or channelscomprising a linear recess in the cooling plate. In some embodiments,this recess may be produced by extrusion, stamping, or forming, e.g.,roll forming. In some embodiments the surface of the cooling plateopposite the jets contains features that mesh and bind with features onthe surface of cooled items, such as 1302. In some embodiments, thenozzles are designed to direct coolant toward areas were the bindingforce or normal force between the cooling plate and the cooled items ishigh.

FIG. 18E is an illustration showing how the cooled components arearranged physically with respect to the patterns of jets in the splitterplate.

FIG. 19 shows a ten-by-ten array of coolant modules 1700 assembled intoan inverter heat exchanger 1900. The rows of coolant modules areterminated in the right back at a cool-coolant manifold 1902, throughwhich cool coolant enters at the port 1904. Manifold 1902 may be locatedat the bottom of the array to facilitate the purging of bubbles andestablishment of uniform flow throughout the array.

The rows of coolant modules are terminated in the left front at ahot-coolant manifold 1906 having an exit port 1908 that may be locatedat the top of the array to facilitate the purging of bubbles. Thisopposing arrangement of inlet and outlet port helps to establish asubstantially uniform flow through the array by creating similarpressure drops in the fluid path.

The use of nozzle orifices and piping that is sized so that the nozzlesproduce the most pressure drop may further assist with maintainingcooling uniformity. The cooling ability of the exchanger may not besubstantially affected by how much liquid is contained in the coolantmodules, but rather by the coolant fluxes through the orifices.

In the event of a coolant pump fault, the volume of coolant in theexchanger may define how long an operator has to repair the pump or turnoff the inverter. This is because the exchanger could switch to aboiling mode.

To maintain a substantially constant coolant pressure, it may be desiredto continue the manifold 1908 or 1902 or both, to a minimum height andterminate the pipe in a free-surface reservoir. Such an arrangement maybe hydrostatically pressurized and resistant to significant overpressures from boiling, pump faults, etc.

The unused bypass ports of the modules can be terminated in plugs. Thisis shown by reference no. 1910 on the front right side, with thecorresponding plug on the back left side being obscured in the viewpresented.

FIGS. 20A-20D illustrate an embodiment of an assembly 2000 of inverterslaves 1500, cooling system 1900, motherboard 2002, and back panel 2004,arranged and fitting together. FIGS. 20A, 20B, 20C, and 20D areillustrations of a front isometric view, a left side view, a right sideview, and a top view of assembly 2000, respectively.

Elements 2006 are connection points for connections with individualclusters. Element 2008 is a three-phase AC connection to go to step-uptransformers. Element 2010 is a connection to the uninterruptible powersupply output.

FIG. 21 shows a mechanical drawing of a housing structure 2100 accordingto an embodiment of the present invention, that contains the assembly2000. This structure comprises a sheet-metal rack enclosure 2102 havingvents 2104 located substantially at the base of the structure, andstandoffs or legs 2106 that are sufficient to work in concert with thevents to provide for relatively unimpeded air flow 2108 to assist withcooling elements of the inverter that are not actively cooled.

The natural convection in such an embodiment may be augmented by the useof a top-mounted vent pipe 2110 having a significant height to producean updraft via the stack effect. Such a pipe may have the additionalbenefit of venting the smell (which may be poisonous) of out-gassingelectronics or failed modules, outside of a secondary inverter enclosuresuch as a shipping container. In some embodiments, natural convectionmay be augmented by the use of an exhaust fan.

FIG. 22 shows assembly 2000 placed in housing 2100 with no front panelattached. The significant open space on the right side may be used tohouse a control computer, batteries for the UPS, UPS control circuitry,coolant pumps, or other elements. In some embodiments, one or more ofthese elements may be designed to reside in a separate container forsafety, corrosion, leakage, safety concerns. FIG. 22 shows andembodiment where the housing 2100 is larger than the assembly 200 in thelateral direction. Other embodiments can include a housing 2100 that islarger than the assembly in the vertical direction or a housing that issubstantially the same size as the assembly. If the housing and theassembly are substantially the same size then elements such as thecontrol computer, batteries for the UPS, UPS control circuitry, andcoolant pumps would be disposed outside of the housing 2100.

FIGS. 23A-23E respectively show a top-front isometric view, a front,back isometric view, a back view, and a side view of the inverterassembly 2300 with front-panel attached. The pipe 2302 is thehot-coolant exhaust port. This pipe should lead to a liquid-air heatexchanger, preferably a radiator having at least the option for forcedconvection.

Alternatively, the radiator may reside at the base of a chimney andderive its cooling in part or entirety from the stack effect. A coolantpipe may rise to a height and terminate in a free-surface reservoir tomaintain a limited pressure head in case of boiling. The pipe 2304 isthe cool-coolant return line. Element 2306 is a digital communicationsconnector, e.g., an Ethernet connector.

FIG. 24 shows an embodiment of the interleaved inverter containing amotorized traverse to programmatically operate arrays of mechanicalswitches. In some embodiments, the traverse may also actuate the lever1501 to disengage inverters. In the embodiment shown, the traverse canbe moved across (2402) and up and down (2404) the inverter front controlpanel. Element 2406 is a traverse stage that may contain a feature orplurality of features to allow it to actuate switches and linear levers.In some embodiments it may be able to actuate rotary switches or screws.In some embodiments, the traverse may be able to produce motion in thedirection normal to the front panel. In some embodiments, an alternativemechanical arrangement of a robot may be employed, e.g., and articulatedarm, etc.

In some embodiments, the inverter change out may be entirely performedby a comparatively unsophisticated robot. In such embodiments, humanoperators may be kept far from high-voltage safety hazards. In someembodiments, inverter consumable replacement and repair may be performedrobotically.

According to certain embodiments of the present invention, the inverterslave self diagnoses faults. In some embodiments, the inverter slave canonly crudely identify that a fault has occurred by not achieving anexpected voltage or current, or by some parameter such as operatingtemperature going outside of an expected range.

In some embodiments, the ability to perform thorough automated troubleshooting may be relegated to test equipment. This test equipment mayplug into the normal inverter connectors and in some cases a specialtest connector providing access to additional signals and controls thatmay aid in diagnosis.

Some embodiments of inverters according to the present invention containsmoke, heat, or fire detectors. Some embodiments may containfire-suppression apparatus, such as CO2, dry chemicals, halon, foam orother fire-suppression means known in the art.

In some embodiments these fire suppressors are under manual control. Insome embodiments, these fire suppressors are under programmatic control.In some embodiments these fire suppressors are controlled by a smoke,heat, or fire detector.

Some embodiments of the present invention contain leak and waterdetectors.

Some embodiments of the present invention may contain certain elementssuch as power filter capacitors within explosion-proof vessels. In someembodiments, capacitors are liquid cooled.

In some embodiments, the coolant comprises dielectric oil. In some suchembodiments, inverter phases may be immersed in circulating coolantrather than isolated from the coolant. According to certain embodiments,inverter slave assemblies are disposed vertically, with controls andindicators facing substantially upward and the remainder of the inverterimmersed in circulating coolant. In some embodiments, coolant iscirculated using a splitter-plate jet architecture as in 1710.

In some embodiments of the present invention, inductors are cooled via awrap-around sink in contact with the cooling plate. In otherembodiments, inductors are cooled via direct impingement or immersion ofa conventional on a conformal insulating surface. In some embodiments,inductors are cooled via direct immersion or impingement of aninsulating coolant.

In certain embodiments, the support structure, the cooling structure,the inverter housing, and the switch modules may contain springs, suchas wavy washers, and the like. These springs function to maintain apreload despite thermal expansion, creep, etc.

Plant Layout

FIG. 3 shows an illustration according to the present invention, of anembodiment of a power plant layout (300). Clusters (302) are arrayed inrows and columns. While site-specific terrain may require a departurefrom a standardized plant plan (such as a uniform array),standardization is advantageous because it permits a maximal amount ofprefabrication of interconnects.

Array rows and columns need not be orthogonal. However, large departurefrom orthogonality may lengthen interconnect lengths appreciably.Individual lines (304, 306) transmit DC power to a massively interleavedcentral inverter (308) that may be located near the center of the arrayto reduce interconnect cost and losses.

A potentially favorable interconnect geometry involves running cablestoward a central point along a row or column, and then running in theother direction toward the array center. This arrangement results ininterconnect lengths that are somewhat longer than more straight-lineconnection, but eases prefabrication and speeds installation whilemitigating potential shading of clusters by the interconnection system.

To reduce inductive and resistive losses, the transformers (310) thatstep the three-phase inverter output to grid (312) voltages are locatedwithin 30 m and even within 10 m of the inverter.

The benefits of interleaving are well known and include lower passivecomponent filter requirements, higher-effective switching frequency,graceful degradation in performance with component failures, etc. Inaddition, the timing of switching can be coordinated to reduce switchingstresses, providing improved switch life and power efficiency.

Co-locating all inverters reduces maintenance costs, provides for fewerenclosures and environmental seals, and eases water cooling, amongstother advantages. Moreover, the physical proximity of invertersfacilitates inexpensive, high-speed timing coordination betweeninverters (for example via clock and hand-shaking lines, high-speedsynchronous and asynchronous communication).

Installation of inverter systems in shipping containers may conferconsiderable cost benefits. Examples of such benefits include ease indistribution, and the advantage of using a commoditized and ruggedized,fire and environment-proof container.

Where a power plant already requires water cooling, the marginal cost ofwater cooling our inverters is small as compared to the advantages inpower density, reduction in noisy fans, and elimination of points ofegress (allowing entry of insects, environmental moisture, and pests).Water cooling inductors, switches, and capacitors may improve lifetimeand performance envelope.

An example of an architecture for each individual inverter, is aboost-type three-phase inverter like that outlined in Y. Chen, K.Smedley, and J. Brouwer (2006) “A Cost-effective Three-phaseGrid-connected Inverter with Maximum Power Point Tracking,” IEEE1-4244-0365-0/06 and Y. Chen and K. Smedley (2006) “Three-PhaseBoost-Type Grid Connected Inverters,” IEEE 0-7803-9547-6/06, both ofwhich are incorporated by reference in their entireties herein for allpurposes. This inverter features a series input inductor, in accordancewith our requirement that the inverter benefit from the inductance ofthe DC interconnects. This inductor sees a relatively low ripplecurrent, reducing the size of the output capacitor needed at the cluster(and/or at receivers). The circuit employs six switches to produce avoltage boosted three-phase output. By carefully time-sequencing theswitches, two thirds of the power switches can be soft-switched and thehard-switching is distributed evenly on the switches. It is anticipatedthat this performance can be further improved by judicious interleavingof many inverters. This inverter was successfully operated using OCCcircuitry. A simple maximum power-point tracking scheme was alsodeveloped. These automatic analog controls contain hooks to allowprecision digital fine-tuning and optimization from relatively modestmicrocontrollers.

While buck inverters achieve higher efficiencies in practice, the use ofa buck inverter requires a minimum operating voltage. Efficientinversion is obtained when this operating voltage is only slightlyhigher than the maximum voltages differentials in the inverted waveform.

However, in accordance with certain embodiments where at any time theremay be a large number of clusters where one or more concentrators areout of commission awaiting service, it may be necessary to operate at acorrespondingly higher cluster voltage or provide a voltage boost stage,both of which reduce the efficiency of the overall system.

In contrast with the buck inverter, the boost inverter performancedegrades gracefully with loss of voltage. Its proper function requiresthat a maximum voltage is not exceeded, which can be accomplished bydesign, given a photovoltaic source.

An inverter system according to an embodiment of the present inventionmay have the ability control the power factor of the plant directly, toassist in maximizing the production value of a power plant. Such may berequired in order to 1) comply with the grid interconnectionrequirements specified by the local Electric Utility, 2) maintainconsistent and stable inverter operations, 3) maximize power productionrevenues (by normally producing at near unity power factor), and 4)provide the opportunity at the plant level to participate in anyavailable ancillary-services markets provided by Utilities orIndependent System Operators.

Power factor control may be achieved at the inverter, as a part of theinverter control, preferably by a mOCC technique. Embodiments of thepresent invention may also exhibit the ability to make cycle-by-cycleand inter-cycle adjustments to output current, to mitigate gridanomalies produced by transient loads and power surges.

FIG. 3A shows a simplified view of a power plant according to anembodiment of the present invention. As shown in FIG. 3A a power plant(350) according to embodiments of the present invention may comprise anarray of clusters (352) comprising a plurality of receivers (354).

The power from each receiver may be maximized by the use of a separatebalancer circuit (356) for each cluster that works by performing a powerconversion only on the deviation of a series-connected receiver from itsmaximum power point. The substantially D.C. output of each clusterconducts along separate twisted pairs of conductors (358) to aninterleaved central inverter (364).

In some embodiments, the inductance of the twisted pairs may beintentionally enhanced by spacing the conductors. In certainembodiments, the inductance of the connections may be intentionallyreduced, for example by breaking the conductors into a plurality ofsmaller, tightly coupled conductors. In particular embodiments, theconductors may be disposed coaxially.

As shown in FIG. 3A, in some embodiments, the cable pairs (358) aregrouped with other cables (360) that run down pairs of rows of clusters.This depiction may indicate placing the cable pairs in physicalproximity, but does not require interconnecting.

Avoiding the combination of the power from numbers of clusters may offercertain potential benefits. One such benefit may be an ability toperform maximum power point tracking for each cluster efficiently. Apossible benefit may be reduced cluster-cluster coupling, such thatproblems affecting one cluster may have a lessened impact on theperformance or health of other clusters. Still another potential benefitmay be that the power and current on each interleaved inverter may bestrictly limited, reducing the need for protection circuitry associatedwith excessive D.C.-side power and current. Yet another possible benefitis that advantages of the use of interleaving may be taken withoutexcessive concern for “load balancing” of the interleaved circuits.

In some embodiments, the diameter of the interconnect wire is chosenindividually depending on the length of the cable run. For example, thediameter may be chosen to obtain a substantially constant resistance ineach cable run, so as to obtain a target D.C. transmission efficiency ator near the minimum required conductor cost.

According to certain embodiments, the balancer circuit acts to establisha cluster voltage to permit interconnection of the cables from aplurality of clusters, without a substantial loss in cluster power-pointmaximization. In some embodiments, a plurality of cluster conductors maybe interconnected. Cables from clusters may become strands of a cable asthey join such a bussed connection in such a way that the diameter ofthe cable grows as the current-bearing requirements grow. In certainembodiments, this optimization of interconnect conductor mass isperformed otherwise.

In some embodiments, cables from only a single row of clusters ratherthan a pair of clusters are grouped. In some embodiments, these groupedpower cables from rows are further grouped with grouped cables fromother rows (362) to converge on a central inverter (364). In someembodiments, this central inverter can be a massively interleavedinverter, with each interleaved inverter circuit fed by one or a subsetof cluster cables.

The central inverter may not employ an isolation transformer. Gridisolation can be provided by stepping up the three phase AC output ofthe inverter (366) via step-up transformer (368) either to distributionor transmission voltages on lines (370).

In a particular embodiment an array may be disposed in a rectangulargrid having ninety-degree angles between cluster axes. Alternatively anarray may be disposed in a parallelogram grid having an angle betweencluster rows and columns other than ninety degrees.

According to certain embodiments of the present invention, this arrayangle is chosen to reduce the impact of self-shadowing on the plantpower output. The calculation of optimal angle may take intoconsideration one or more factors, including but not limited to thelocal latitude, the lay of the land, details of the cluster design,local weather patterns, utility power pricing schedules, land costs peracre, parcel boundary, and others.

The East-West spacing of clusters (372) may be different from theNorth-South spacing of clusters (374), according to an optimizationcalculation that involves substantially the same parameters as thecalculation of an optimal array angle. In some preferred embodiments,this spacing ratio is approximately 2:1. In other preferred embodiments,this spacing ratio may vary from approximately 1.3:1 to 3.5:1.

In some embodiments of the present invention, parameters including therow and column spacings and array angle may vary across a plant toaccommodate un-even ground or flora. According to certain embodiments,the location of clusters may be intentionally staggered to achieve aparticular appearance, including a desired aesthetic appearance, apixilated image comprising clusters as pixels, a geometric pattern, arandom or quasi-random pattern, or others.

In whatever form the power plants are arrayed, it may be advantageous todenumerate clusters according to array indices, e.g., {m,n}. As usedherein, the term ‘row’ is defined as a set of clusters having a givenconstant index m, i.e. mth row, and the term ‘column’ is defined as aset of clusters having a given constant index n, i.e., the nth column.

In some embodiments of the present invention, a “central inverter” maybe located near the center of an array. If a plant is large, such thatthe interconnect losses or cost between clusters on the periphery andcenter of the plant is high), the plant may be subdivided into aplurality of sub-arrays, each having an inverter located substantiallycentrally to the sub-arrays. Herein, the term “array” can refer to sucha sub-array.

In some embodiments, power from a cluster may be conducted on asubstantially balanced pair of conductors. In some embodiments, theseconductors may be disposed helically about each other to reduceemissions. In some embodiments, power from a cluster may be conducted onan unbalanced, coaxial line. In some embodiments, the conductors arespaced to increase the inductance of the interconnects. In someembodiments, the conductors are spaced only by a minimal insulator toreduce inductance. In some embodiments, inductance is further reduced bydividing the cables into a plurality of parallel lines and interspersingthe cables.

FIGS. 3AA-3AD show arrangements of cluster voltages according toembodiments of the present invention. In the arrangement 380 in FIG.3AA, the positive voltage from a cluster, VC+ (382), can vary accordingto the power output of the cluster, while the negative voltage from acluster, VC− (384), remains substantially at a zero-Volt reference(e.g., earth ground). According to certain embodiments, this constantvalue is established at the central inverter. In some embodiments, thisconstant value is established at the cluster.

Arrangement 390 in FIG. 3AB is the same as arrangement 380 except thatVC− (392) is allowed to vary while VC+ (394) remains substantially at azero-Volt reference.

Some embodiments comprise a combination of cluster cables according toarrangement 380 and 390.

Arrangement 396 in FIG. 3AC is similar to 380, but VC− (398) ismaintained at a substantial negative value with respect to ground 397.This type of arrangement may be beneficial for allowing high clustervoltage differences [(VC+)−(VC−)] to be conducted safely andinexpensively through a plant.

Arrangement 399 in FIG. 3AD is similar to 396, but with the oppositepolarity.

Arrangement 386 in FIG. 3AE maintains substantially symmetric voltages387 and 388 about a reference voltage 389 (e.g., 0 V or earth ground).In some embodiments, this substantially symmetrical variation ismaintained through a control loop in an inverter circuit. In otherembodiments, this symmetry is maintained at the cluster. According tocertain embodiments, this arrangement may be maintained by connectingapproximately equal numbers of receivers in series on the positive andnegative side of earth ground near a cluster.

The absolute magnitude of the difference [(VC+)−(VC−)] can vary widely,depending on the state of health of a cluster. For example, thisabsolute magnitude can vary depending upon whether receivers arenon-functioning or poorly functioning, the amount of illumination on thereceivers, and other factors.

The clusters should tolerate the variation in this cluster voltage. Forexample any internal power supplies derived from the cluster voltage,should be able to tolerate a sufficiently wide variation.

For cost effectiveness and efficiency, some power-intensive deviceswithin a cluster, such as motors and pumps, may be powered via switchinga voltage waveforms from the cluster bus directly. These devices shouldbe able to perform some operations at a minimum cluster voltage.

Particular embodiments of the present invention may extract powerefficiently from clusters comprising concentrated photovoltaicreceivers. In some embodiments, the concentrators are subject to routinemaintenance and replacement.

Accordingly, in certain embodiments it may be desirable to tolerate apopulation of un-producing receivers to stabilize the rate ofmaintenance. It may be desirable to extract power from functioningreceivers in spite of one or a plurality of un-producing receiverswithin a cluster.

In some embodiments, the efficiency with which power is extracted maydrop with the number of functioning receivers, in exchange for improvedefficiency with all or all-but-one receiver functioning. For example,this efficiency compromise may favor the use of a boost stage in aninverter when one or more than one receivers is not producing.

Greater numbers of non-producing receivers may require higher levels ofboost, thereby reducing efficiency. Some embodiments can produce amodest boost, e.g., 1.5:1. Other embodiments may produce a significantboost, e.g., up to ˜3:1. Still other embodiments allow a higher (in somecases arbitrary) voltage boost, and are switched off only when prudent.For example, certain embodiments may switch off when the net extractedpower equals zero. In some embodiments, when a cluster is completely ornearly completely functioning, the boost stage may be switched out ornot operated, providing high inverter efficiency.

The inverter connected to the cluster can likewise tolerate a largevoltage swing. For this reason, some embodiments may utilize avoltage-boosting inverter. In certain embodiments, the invertercomprises a boost stage followed by a buck inverters.

According to some embodiments, the inverter comprises a boost stage thatoperates only when necessary followed by a buck inverter. Suchembodiments may enjoy high efficiency in good illumination and theability to produce power in less than ideal illumination at a slightlyreduced efficiency.

When there is insufficient production from the clusters to powerthemselves, the inverter may supply auxiliary power to a cluster bymaintaining a nominal cluster voltage.

In some embodiments, a plurality of converters may be utilizedintermediately between groups of clusters and the central inverter. Theconverters may act on imbalance voltages between clusters, allowingcombination of cluster outputs by paralleling while maintaining maximumpower-point tracking of individual clusters. The converters may act onimbalance current between clusters, allowing combination of clusteroutputs by series connection.

In some embodiments, outputs of a plurality of clusters (for example arow of clusters or a column of clusters) may individually feed aninterleaved inverter. A plurality of outputs from inverters may becombined centrally and stepped up to grid voltage or stepped upindividually and combined centrally.

In some embodiments of the present invention, the central invertersolely establishes the common potential for the power system of theplant. This common potential is commonly called “ground,” but is not tobe confused with the “earth ground” potential, because the localpotential of the earth may vary considerably across the extent of apower plant. Moreover, the potential established at the central invertermay be different from that at a cluster as a result of ohmic losses inthe current return line.

Interconnects

Relatively long transmission-line runs (such as O (1 km)) may beunavoidable in designing plants that harvest appreciable amounts ofsolar power. Accordingly it may be important to 1) control the cost ofthe interconnect wires, 2) minimize conductance losses, 3) controlEMI/RFI, and/or 4) control inductance.

Material selection may be important. From a cost standpoint, Aluminumwire is favored, costing several times less than a copper conductor ofthe same resistance. For long runs, difficulties connecting aluminumwires are far outweighed by the cost savings.

Material cost should be minimized by judiciously choosing conductor sizefor a substantially consistent resistance from each cluster to theinverter. In most cases, overall cost is reduced at a small decrease inmaterial efficiency by selecting the wire gauge from a limited number ofdiscrete sizes.

FIG. 4A shows a diagram of a pair of DC transmission lines (400). Wire402 carries current of one sign; wire 404 carries a return current. Bytwisting the wires, as known in the art, electromagnetic field emissionsaway from the wires are significantly reduced as is pickup of noise fromnearby cables.

In addition to twisting the cables with a pitch (410), the wires canbeneficially have a gap (408) between them. The wires then behavesimilarly to an air-spaced parallel-sided wire loop, provided the helixangle is small and the spacing between the wires is several wirediameters (406). With a modest air gap, e.g., 3 to 50 mm, especially ˜10mm, such air-spaced wires posses a considerable inductance over typicalplant-scale runs, e.g., O (100 uH). A low ratios of helix length tospacing, significantly more conductor is needed to cover a lineardistance than with an un-twisted wire pair, but EMI/RFI shielding isbetter. Favorable ratios are between 1:1 and 50:1, with ˜12:1 being areasonable compromise.

Being an air-spaced inductor, the inductor losses should be low,provided these cables are kept from low-resistance ferrometals byseveral pitch distances (410) to avoid hysteresis and eddy-currentlosses. This series inductance can be used to enhance the operation ofproperly architected inverters, such as the boost inverter describedabove. In some cases, it may be possible to eliminate discrete powerinductors in these inverters.

In some cases it is advantageous to flatten the helix. Such aconfiguration may be used to allow the cable to roll up or flex morereadily in one direction.

As illustrated in the embodiment of FIG. 4B the spacing of an insulatedtwisted pair (420) may be maintained by a polymer web (422) having across-section like that of 424, which minimizes polymer use. Thisinsulator may be extruded with bare wires or pre-insulated wires. Asufficiently inexpensive superparamagnetic, magnetic, or ferromagneticpowder, such as hematite, Fe₂O₃, iron powder, iron-silicon powder, etc.could be mixed with the spacer polymer such that the spacer and/or wireinsulators enhance the inductance of the transmission wires. This wouldfurther reduce emissions and improve inverter performance.

The embodiment of FIG. 4C shows an alternative spacer cross-section(444) that may be adopted if the use of an inductance-modifying fillermaterial is cost justified.

The following summarizes one or more characteristics of particularembodiments of an inverter.

-   -   A boost architecture may be favored so there is tolerance for        one or more concentrators in a cluster to be out of service with        a modest loss of efficiency. Moreover a boost architecture        allows a plant to operate in relatively low direct normal        irradiance.    -   Water cooling may be used liberally to enhance component life,        reduce size and mass, eliminate costly heat sinks, etc.    -   The standard enclosure for the inverter may be a shipping        container into which inverter racks are integrated in a factory.    -   Power from individual clusters or small groups of clusters may        be inverted individually in a massively interleaved array, e.g.,        O (200 inverters) per megawatt.    -   Interleaved switching may be coordinated to reduce switching        stresses and improve efficiency while reducing RFI and        line-voltage distortion.    -   Components that are subject to failure (such as switches and        their drivers, capacitors, etc.) may be individually packaged to        minimize the cost of the consumable element and the maintenance        cost of its replacement.    -   Normal failures shall degrade performance gracefully.    -   Imminent failure and cumulative damage may be monitored if        practical to support preventive maintenance.    -   Secondary damage from normal failures may be prevented by        appropriate design of (potentially reusable) individual        containers.    -   Consumable component containers may also contain features for        mechanical locking, making electrical connections, etc., to        assist with simple change out. Some of these connections can be        used to detect the absence or replacement of the device.    -   These component containers may further act as adapters to allow        us to replace failed components with new ones having different        physical geometries as components evolve.    -   Maintenance should not require specialized knowledge or skills.        Replacing consumables should be safely performed hot and, to the        greatest extent practical, returning inverters to service should        be nearly automatic.    -   The state of health and performance of all circuitry can be        monitored, controlled, and troubleshot/debugged using our        standard RS485/Ethernet network.    -   All firmware can be safely upgraded remotely using our standard        RS485/Ethernet network.

Communications and Control

FIG. 25 shows a schematic diagram of a communications and controlnetwork 2500 according to an embodiment of the present invention. Achallenge with a large-area plant is to maintain tight control to detectand handle faults, and/or to sense and optimize performance, in spite ofthe potential for intense electrical noise and/or substantialground-level variations.

In some embodiments, communications may be handled in part by a “Rowbridge” (2502) that serves as a bridge between a higher-level, in somecases, plant-wide communications network (2504) such as an Ethernetnetwork, RS485 network, or other differential voltage signaling networkand a sub network that services one or two rows of clusters.

In some embodiments, sub network communications utilizes a pair oftwisted pair cables, 2506 and 2508. In some embodiments, these cablescomprise a twisted ribbon cable. In some embodiments, these cablescomprise separate twisted wires, such as those in CATS or telephonycables.

In some embodiments, at least cable 2508 has a controlled impedance.Examples of such controlled impedance include between about 30-300Ω, andbetween about 80-120Ω.

In some embodiments, the row sub network utilizes the RS485 protocol. Insome embodiments the Row bridge establishes a low-voltage communicationbus power, for example between about 5 to 24 VDC, that provides power tothe sub network side of a bridge 2510 between the sub network and one ormore cluster networks. This power is used by nodes on the sub network totransmit differential voltages on the twisted pair 2508. The power mayalso provide a voltage reference to ensure that the common-modetransmitted voltage lies within the range of the Row-bridge (2502)receiver.

The particular embodiment of FIG. 25 shows the one-or-more clusterbridge as being a bi-cluster bridge. The one-or-more cluster bridgeprovides voltage isolation between the sub network (2508) and thecluster network (2516). Element 2512 depicts a cluster.

According to certain embodiments, it may be desirable to supply primarypower to the bridge from one or more of the connected clusters (e.g.,via 2514) rather than 2506. This is because the run length of 2506 maybe much longer, and that power may be shared by a plurality of clusterbridges.

If the bridge serves more than one cluster, it may need to isolatecommunications lines (2516) and (2518) between the two clusters. This isbecause in some embodiments, clusters may have ground references thatvary with respect to each other by more than the common-mode rangeafforded by the RS485 or other differential signaling specification.This ground-reference variation may be influenced by possibledifferences between the cluster voltages (2520 and 2522).

According to certain embodiments it may be desirable to arrange thecommunications line 2508 as a transmission line having negligibledistance 2524 between the line and cluster bridges. The cluster bridgesin turn may reside at a node in a cluster network transmission line.

Communications transmission line terminals can be properly terminated inan appropriate impedance to prevent signal degradation from reflections.Reliable communications rates >1 MBAUD may be achieved in such a system,even for a plant having a span of order of 1 km. Such a communicationrate may support advanced plant-power optimization, even with rapidvariations in illumination and collection efficiency (for example fromwind gusts and buffeting).

Plant Power Usage

The primary power used by clusters for their operations (e.g.communications, cooling, pointing, tracking, focusing, and others) maybe supplied via the cluster voltage difference [(VC+)−(VC−)]. Duringproduction, this voltage difference may be produced by the receivers.During periods of no production, this voltage difference may be suppliedfrom the central inverter, which normally draws this auxiliary powerfrom the grid.

In certain embodiments, the inverter may have a modest battery toprovide and sink bus power momentarily in the event of a grid outage.This battery may be backed up by a diesel generator to provide plantpower during extended outages during which no production is possible.

In the event of a forced grid disconnection while producing power, thebattery or other load may provide a momentary power sink while the planttakes steps to reduce power output. In some embodiments, clusters mayshort-circuit one or more receivers to prevent an excessive rise in busvoltage

In summary, embodiments of power system architectures in accordance withthe present invention may be determined by one or more factors. Onefactor is the nature of the basic power-producing unit. In certainembodiments, this basic power-producing unit may comprise a siliconsolar cell that produces only ˜0.6 V and has a particular maximum powerpoint of operation that depends on the device and the level ofillumination.

Another factor is the nature of the power resource. In solar powerembodiments, sunlight is a dilute power resource, and therefore powermay need to be collected over large areas and transmitted long distancesin a solar power plant.

Still another factor is the maximum voltage. For safety of maintenancestaff, a maximum voltage of electrical transmission within a powerplant. This maximum may also be set by device economics, and in somecases by device physics.

Still other factors comprise practical constraints. For example, in asystem based upon photovoltaic receivers, it may be expensive anddifficult to ensure that these receivers share a common maximum-powercurrent without active electronic intervention.

The above considerations may lead to embodiments of plant architecturesexhibiting one or more of the following characteristics. First, cellsmay be series connected in correlated strings (for example a module,dense-array receiver, or ganged sparse array), which are genericallyreferred to as receivers.

It may be difficult to perform maximum power point tracking for seriesstrings containing fewer than a threshold number of cells. Below thisthreshold level, passive optical and electrical techniques may be neededto balance currents. Examples of such techniques include but are notlimited to, judicious paralleling of compensating substrings, in orderto make departures from maximum power point operation acceptably low.

Above this threshold number of cells, active circuitry can beeconomically employed to improve efficiency. However, this circuitry mayoperate only on “imbalance” currents. That is, receivers can be seriesconnected in a “cluster” to boost the voltage. Active circuitry can onlysupply deficit or drain surplus currents to and from receivers such thateach receiver operates near its maximum power point.

It is possible to group these clusters of receivers into super-clustersand perform the same balancing operation between clusters. However,adding layers to this hierarchy can increase costs.

Certain embodiments may employ one or two balancing stages for oursystem. The last stage of this balancer may comprise a sufficient numberof series cell connections to reach the maximum bus voltage deemed safefor intra-plant transmission.

In certain embodiments, power may be transmitted to a single point(which may be at or near a center of a power plant), over separate,spaced twisted wires specifically optimized for increased inductance anda target resistive loss at maximum current. The length of theseinterconnect wires can be O (100 m to 1 km) depending on the powerrating of the plant.

A separate three-phase boost inverter can be connected via these wiresseparately to each cluster. That is, the power-bearing wires may not bebussed on their way to the inverter. Keeping these wires separate helpsto avoid issues of maximum-power-point mismatch at the combination pointand inverter load balancing at the splitting point.

In certain embodiments the boost inverters may be massively interleaved.Such configurations may serve a number of purposes, including reducingfiltering requirements, and reducing EMI/RFI.

Massive interleaving of the boost inverters may permit expansion of thepeak efficiency envelope. For example, such configurations may allowvariable frequency operation. In some embodiments the outputs of theinverters may be directly connected, e.g., without an isolationtransformer.

According to certain embodiments, the inverter interleaving can bedeterministically coordinated, allowing optimizations such assoft-switching. In particular, filtering requirements of a deterministicsystem ideally scale as N⁻¹, where N is the number of inverters.

Above a threshold number (N_(d)), non-idealities reduce theeffectiveness of the interleaving and further decreases in filteringrequirements are negligible. Above another threshold (N_(nd))non-deterministic interleaving, which reduces filtering requirements asN^(−1/2), is cost effective and readily supports “one-cycle control,” aninexpensive, high-performance control technique.

In certain embodiments, the outputs of the interleaved inverter arraymay be boosted directly to grid voltages via transformers located within˜10 m of the inverter array. Some embodiments may employ water coolingfor increased cooling performance, reduced cost, and reduced parasiticload.

While the above-referenced embodiments have been described in connectionwith the design of plants utilizing energy conversion devices in theform of inflated balloons for collecting solar energy, the presentinvention is not limited to this approach. In accordance withalternative embodiments, similar considerations may apply to alternatearrays of energy conversion devices, including but not limited tochemical cells, batteries, thermocouples, fuel cells, wind turbines, andwater turbines.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations, andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited to the specificembodiments as illustrated herein, but is only limited by the followingclaims.

What is claimed is:
 1. A method for balancing circuit voltagecomprising: receiving, at a circuit, power from a plurality ofpower-generating elements, the circuit being coupled to first and seconddevices, the first device being communicatively coupled to the seconddevice; and increasing, using a switch of the second device, a load onat least one of the plurality of power-generating elements such that thevoltage of the circuit does not exceed an upper threshold voltage. 2.The method of claim 1, wherein the first device is an inverter and thesecond device is a balancer.
 3. The method of claim 1, wherein theswitch is a semiconductor or mechanical relay.
 4. The method of claim 1,wherein the circuit includes a pair of wires that are spaced a firstdistance apart and are helically twisted.
 5. The method of claim 4,wherein the circuit further includes an insulator disposed between thepair of wires.
 6. The method of claim 4, wherein the first distance isgreater than a diameter of each of the pair of wires.
 7. The method ofclaim 6, wherein the helically twisted pair of wires is flattened.